Compressive strength of concrete and mortar containing fly ash

ABSTRACT

The present invention relates to concrete, mortar and other hardenable mixtures comprising cement and fly ash for use in construction. The invention includes a method for predicting the compressive strength of such a hardenable mixture, which is very important for planning a project. The invention also relates to hardenable mixtures comprising cement and fly ash which can achieve greater compressive strength than hardenable mixtures containing only concrete over the time period relevant for construction. In a specific embodiment, a formula is provided that accurately predicts compressive strength of concrete containing fly ash out to 180 days. In other specific examples, concrete and mortar containing about 15% to 25% fly ash as a replacement for cement, which are capable of meeting design specifications required for building and highway construction, are provided. Such materials can thus significantly reduce construction costs.

The research leading to the present invention was conducted withGovernment support under Contract No. DE-FG22-90PC90299 awarded by theDepartment of Energy. The Government has certain rights in thisinvention.

FIELD OF THE INVENTION

The present invention relates to concrete, mortar and other hardenablemixtures comprising cement and fly ash for use in construction. Theinvention includes a method for predicting the compressive strength ofsuch a hardenable mixture, which is very important for planning aproject. The invention also relates to hardenable mixtures comprisingcement and fly ash which can achieve greater compressive strength thanhardenable mixtures containing only concrete over the time periodrelevant for construction.

BACKGROUND OF THE INVENTION

Fly ash, a by-product of coal burning power plant, is produced worldwidein large quantities each year. In 1988, approximately 84 million tons ofcoal ash were produced in the U.S. in the form of fly ash (60.7%),bottom ash (16.7%), boiler slag (5.9%), and flue gas desulfurization(16.7%) (Tyson, 1990, Coal Combustion By-Product Utilization Seminar,Pittsburgh, 15 pp.). Out of the approximately 50 million tons of fly ashgenerated annually, only about 10 percent is used in concrete (ACICommittee 226, 1987, "Use of Fly Ash In Concrete," ACI 226.3R-87, ACI J.Proceedings 84:381-409) while the remaining portion is mostly disposedof as waste in landfills.

It is generally more beneficial for a utility to sell its ash, even atlow or subsidized prices, rather than to dispose of it in a landfill,since this will avoid the disposal cost. In the 1960's and 70's the costof ash disposal was typically less than $1.00 per ton. However, due tothe more stringent environmental regulations starting in the late1970's, the cost of ash disposal has rapidly increased to from $2.00 to$5.00 per ton and is still rising higher (Bahor and Golden, 1984,Proceedings, 2nd International Conference on Ash Technology andMarketing, London, pp. 133-136). The shortage of landfill due toenvironmental concerns has further escalated the disposal cost. TheEnvironmental Protection Agency (EPA) estimated in 1987 that the totalcost of waste disposal at coal fired power plants ranged from $11.00 to$20.00 per ton for fly ash and bottom ash (Courst, 1991, Proceedings:9th Int'l 1.0 Ash Use Symposium, 1:21-1 to 21-10). This increasing trendof disposal cost has caused many concerns and researchers are urgentlyseeking means for better utilization of fly ash. One potential outletfor fly ash is incorporation in concrete or mortar mixtures.

Fly ash is used in concrete in two distinct ways, one as a replacementfor cement and the other as a filler. The first use takes advantage ofthe pozzolan properties of fly ash, which, when it reacts with lime orcalcium hydroxide, can enhance the strength of cementitious composites.However, fly ash is relatively inert and the increase in compressivestrength can take up to 90 days to materialize. Also, since fly ash isjust a by-product from the power industry, the quality of fly ash hasalways been a major concern to the end users in the concrete industry.

Incorporation of fly ash in concrete improves workability and therebyreduces the water requirement with respect to the conventional concrete.This is most beneficial where concrete is pumped into place. Amongnumerous other beneficial effects are reduced bleeding, reducedsegregation, reduced permeability, increased plasticity, lowered heat ofhydration, and increases setting times (ACI Committee 226, 1987, supra).The slump is higher when fly ash is used (Ukita et al., 1989, SP-114,American Concrete Institute. Detroit, pp.219-240).

However, the use of fly ash in concrete has many drawbacks. For example,addition of fly ash to concrete results in a product with low airentrainment and low early strength development.

As noted above, a critical drawback of the use of fly ash in concrete isthat initially the fly ash significantly reduces the compressivestrength of the concrete. Tests conducted by Ravindrarajah and Tam(1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete,SP-114, American Concrete Institute, Detroit, pp. 139-155) showed thatthe compressive strength of fly ash concrete at early ages are lowerthan those for the control concrete, which is a general property ofconcrete or mortar when fly ash is added. Most of the reported studiestend to show a lower concrete strength due to the presence of fly ash;none has yet suggested a solution to actually enhance the property ofconcrete economically. Yet, for fly ash to be used as a replacement forcement, it must be comparable to cement in terms of strengthcontribution at a point useful in construction. As a practical matter,this means that the fly ash concrete must reach an acceptablecompressive strength within about 2 weeks.

Swamy (1984, Proceedings, 2nd Int'l Conference on Ash Technology andMarketing, London, pp. 359-367) showed that 30% replacement by weight,and inclusion of a high dose of a superplasticizer, yielded concretewith material properties and structural behavior almost identical tothose of concrete of similar strength without fly ash. However, due tothe high cost of superplasticizer, mix proportions were not economical.

Fly ashes from different sources may have different effect to concrete.The same fly ash may behave differently with portland cements ofdifferent types (Popovics, 1982, ACI J. Proceedings 79:43-49), sincedifferent types of portland cement (type I to V) have different chemicalcomposition. Other factors relating to the effects of fly ash onconcrete that are not presently understood are lime availability, therate of solubility and reactivity of the glassy phase in different flyash, and the proper mix proportion to ensure early strength developmentof fly ash concrete.

Fly ash particles are typically spherical, ranging in diameter from 1 to150 microns (Berry and Malhotra, 1980, ACI J. Proceedings 77:59-73).Aitcin et al. (1986, Fly Ash, Silica Fume, Slag, and Natural Pozzolansin Concrete, SP-91, American Concrete Institute, Detroit, pp. 91-113)showed that if the average diameters, D₅₀, of fly ash are smaller, thesurface area of the fly ash will be larger than those with largeraverage diameters.

Many factors affect the size or average diameter of fly ash, includingstorage conditions, ash collection processes, and combustion conditions.Combustion conditions are perhaps most important, because thesedetermine whether carbon remains in the ash or if combustion iscomplete.

There are two main forms of combustion: dry bottom boiler combustion andwet bottom boiler combustion. The main difference between the two typesof boiler is that wet bottom boilers reach the fusion temperature ofash, thus resulting in fly ash with greater glass characteristics.

There are generally two methods known to measure the fineness of flyash. The first is by measuring the residue on the 45 micron (No. 325sieve), which is the method used in the United States. The second methodis the surface area method by air permeability test. Lane and Best(1982, Concrete Int'l: Design & Construction 4:81-92) suggested that 45microns sieve residue is a consistent indicator of pozzolanic activity.For use in concrete or mortar, ASTM C 618 (1990, ASTM C 618-89a, AnnualBook of ASTM Standards, Vol. 04.02) specifies that not more than 34% byweight of a given fly ash be retained on a 45 microns sieve. However,Ravina (1980, Cement and Concrete Research 10:573-580) reported thatspecific surface area provides a more accurate indicator of pozzolanicactivity.

Research carried out by Ukita et al. (1989, supra) purported that as thepercentage of finer particles, i.e., those particles ranging fromdiameters of 1 to 20 microns, in concrete increases, the correspondingstrength gain is notable. Similar observations have been reported byGiergiczny and Werynska (1989, Fly Ash, Silica Fume, Slag, and NaturalPozzolans in Concrete, SSP-114, American Concrete Institute, Detroit,pp. 97-115).

Both of the groups mentioned above describe results with fly ash ofdisparate characteristics and sources, but did not include controls forthese variable. Thus, although the emphasis of these reports is on theperformance of finer particle fly ashes, the variables introduced intothe studies lead to reservations with respect to any conclusions thatmay be drawn. In particular, Ukita et al. (1989, supra) collected flyash from different locations. However, an earlier report demonstratedthat fly ashes collected from different locations have differentchemical properties (Liskowitz et al., 1983, "Sorbate Characteristic ofFly Ash," Final Report. U.S. Dept. of Energy, Morgantown EnergyTechnology Center, p. 211 ). Giergiczny and Werynska (1989, supra)ground the original fly ash into different sizes. Grinding can add metalparticles into the fly ash, and also tends to yield unnaturally shapedparticles of fly ash. Thus, these reports fail to provide conclusiveinformation about the effect of fine particle size on the propertiesimparted by fly ash.

Berry et al. (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans inConcrete, SP-114, American concrete Institute, Detroit, pp. 241-273)studied the properties of fly ash with particle size smaller than 45microns, so called "beneficiated" fly ash, in mortar. Fly ashes of thisparticle size showed improved pozzolanic activity, reduced water demandand enhanced ability to reduce alkaliaggregate reactivity.

Although beneficiated fly ash seem to show promising results in terms ofimproved performance of mortar, other researchers concluded otherwisewhen used in concrete. Giaccio and Malhotra (1988, Cement, Concrete, andAggregates 10:88-95) also conducted the test using the beneficiated flyashes. They showed that the concrete made with ASTM type I cement, theuse of beneficiated fly ash and condensed silica fume did little toenhance the properties of concrete compared with the raw fly ash.

It is critically important in construction to have concrete or mortarthat predictably achieves required performance characteristics, e.g., aminimum compressive strength within 14 days. A corollary is that aconstruction or civil engineer must be able to predict the compressivestrength of a concrete or mortar mixture after a given period of time.However, the prior art concrete or mortar mixtures that contain fly ashlack predictability with respect to compressive strength, and generallyhave lower compressive strength than concrete or mortar mixtures thatlack fly ash. Therefore, there has been a disincentive to use fly ash insuch hardenable mixtures.

Thus, there is a need in the art for a method of quantitativelydetermining the rate of strength gain of a concrete or mortar containingfly ash.

There is a further need in the art for high strength concrete and mortarcontaining fly ash.

There is yet a further need in the art for the utilization of fly ashgenerated during coal combustion.

The citation or identification of any reference in this applicationshall not be construed as an admission that such reference is availableas prior art to the present invention.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method forpredicting the compressive strength of a hardenable mixture containingcement and fly ash of a defined fineness comprising determining thecontribution to compressive strength of

1. the compressive strength contributed by the cement over a givenperiod of time, which is a function of the concentration of cement; and

2. the compressive strength contributed by the fly ash of a definedfineness over a given period of time, wherein the fineness is either adistribution of fly ash particle sizes or a distribution of fly ashparticle volumes.

According to the invention, the compressive strength contributed by thefly ash of a defined fineness is a function of the fineness of the flyash, the concentration of fly ash in the mixture, and the age of thehardenable mixture in days.

One measure of fineness of the fly ash is referred to herein as thefineness modulus, which is a measure of the distribution of particlesizes (e.g., diameter) or the distribution of particle volumes. In aspecific embodiment, the fineness modulus is the summation of thepercentage of fly ash that retains on more than one sieves of differentsizes ranging from about 1μ to about 300μ.

A particular advantage of the present invention is that in a preferredaspect it provides a highly quantitative measure of fineness of fly ash,which measure can be used to accurately predict the compressive strengthof a hardenable mixture at a given time.

In a specific embodiment, the compressive strength of the hardenablemixture is determined as a percentage compressive strength of thehardenable mixture compared to a control hardenable mixture that doesnot contain fly ash. In a more particular aspect, the percentagecompressive strength, σ(%), is calculated according to the followingformula:

    σ(%)=0.010C.sup.2 +A+(B/FM)ln(T),

wherein C is the percentage of cement in cementitious materials presentin the hardenable mixture, which cementitious materials include cementand fly ash; A is a constant for the contribution of fineness of fly ashto the strength of the hardenable mixture: B is the constant forpozzolanic activity rate between fly ash and cement, which isproportional to the content of fly ash in the mixture; FM is thefineness modulus of the fly ash, which is the summation of thepercentage of fly ash that retains on more than one sieves of differentsizes ranging from about 1μ to about 300μ; and T is the age of thehardenable mixture in days, wherein T ranges from 1 day to about 180days. In a specific embodiment, infra, the formula was used toaccurately predict compressive strength at various time points up to 180days.

In specific embodiments, the fly ash is either wet bottom boiler fly ashor dry bottom boiler fly ash, and

    A=6.74-0.00528FM.

In other embodiments, the fly ash content of the hardenable mixture isbetween about 10% to about 50% by weight of cementitious materials inthe mixture, and

    B=(1685+126C-1.324C.sup.2).

In a preferred aspect of the invention, the fly ash is wet bottom boilerfly ash or dry bottom boiler fly ash, the fly ash content of thehardenable mixture is between about 10% and about 50%, and

    σ(%)=0.010C.sup.2 +(6.74-0.00528FM)+{(1685+126C-1.324C.sup.2)/FM}In(T).

In a further aspect, the present invention provides hardenable mixturescontaining cement and fly ash that has been fractionated into a definedfineness. The hardenable mixtures of the invention advantageously havepredictable compressive strengths. Preferably, the hardenable mixturesof the invention have the same or greater performance characteristics,such as compressive strength after 7 to 14 days of hardening, as acomparable hardenable mixture that does not include fly ash. Hardenablemixtures of the invention with enhance performance characteristicscomprise fly ash characterized by a distribution of particle sizes orparticle volumes that is less than the median or average fornon-fractionated fly ash. Hardenable mixtures according to the inventioninclude, but are not limited to, concrete and mortar.

Accordingly, the present invention particularly relates to a concretecomprising about 1 part by weight cementitious materials, about 1 toabout 3 parts by weight fine aggregate, about 1 to about 5 parts byweight coarse aggregate, and about 0.35 to about 0.6 parts by weightwater, wherein the cementitious materials comprise from about 10% toabout 50% by weight fly ash and about 50% to about 90% by weight cement,wherein the fly ash has a fineness modulus of less than about 600,wherein the fineness modulus is calculated as the sum of the percent offly ash retained on sieves of 0, 1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150,and 300 microns. Preferably, the fly ash is wet bottom boiler fly ashhaving a fineness modulus of less than about 350 as calculated above.

In a further embodiment, the invention relates to a mortar comprisingabout 1 part by weight cementitious materials, about 1 to about 3 partsby weight fine aggregate, and about 0.35 to about 0.6 parts by weightwater, wherein the cementitious materials comprise from about 10% toabout 50% by weight fly ash and about 50% to about 90% by weight cement,wherein the fly ash has a fineness modulus of less than about 600,wherein the fineness modulus is calculated as the sum of the percent offly ash retained on sieves of 0, 1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150,and 300 microns. Preferably, the fly ash is a wet bottom boiler fly ashhaving a fineness modulus of less than about 350, as calculated above.

As can be appreciated from the foregoing, the present inventionadvantageously provides hardenable mixtures in which fly ash, a veryinexpensive material, replaces cement in the cementitious materials,substantially decreasing the cost of the hardenable mixture withoutsacrificing performance characteristics. In a further aspect, thepresent invention provides hardenable mixtures with enhanced performancecharacteristics at a lower price.

The invention further advantageously provides concrete and mortarmixtures comprising fly ash that do not require an expensivesuperplasticizer. Prior art mixtures require superplasticizer permit areduction in the amount of water in the mixture, thus compensating forthe decrease in compressive strength of the mixture due to addition ofthe fly ash. Thus, the invention provides concrete or mortarsubstantially lacking a plasticizer.

According to the invention, the fine aggregate used in the cement or themortar can comprise a sand and a fly ash, wherein a ratio by weight ofsand to fly ash is from about 4:1 to about 1:1, and the fly ash has afineness modulus of less than about 600, wherein the fineness modulus iscalculated as the sum of the percent of fly ash retained on sieves of 0,1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150, and 300 microns.

In a further aspect, fly ash can be used as an additive in a hardenablemixture, wherein the ratio of additive fly ash to cement of ranges fromabout 1:10 to about 1:1, and wherein the ratio of the total amount offly ash (whether included as a cementitious material, a fine aggregatesubstitute, or as an additive) ranges from about 1:5 to about 2:1.Preferably, the fly ash has a fineness modulus of less than about 600,wherein the fineness modulus is calculated as the sum of the percent offly ash retained on sieves of 0, 1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150,and 300 microns.

Thus, the invention provides for use of fractionated fly ash of adefined fineness to replace cement in the cementitious materials of ahardenable mixture, to replace sand or other fine aggregate of ahardenable mixture, or as an additive, which mixtures have predictableperformance characteristics, demonstrate performance characteristicsthat meet or exceed the standards required for construction, and costsignificantly less than equivalent compositions that lack fly ash. Theinvention further provides a method for predicting the compressivestrength of such hardenable mixtures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents graphs showing the size distribution of fractionated flyash particles and cement particles (inverted triangles, 98% of whichhave a diameter of 75μ or less). (A) Dry bottom boiler fly ash (solidsquare, in which 92% of the particles have a diameter of 75μ or less)and fractions 1C (solid triangle, 95% less than 150μ), 11F (soliddiamond, 96% less than 30μ), 10F (open square, 94% less than 20μ), 6F(open diamond, 99% less than 15μ), 5F (X, 98% less than 10μ), and 3F(open triangle, 90% less than 5μ). (B) Wet bottom boiler fly ash (opensquare, 95% less than 75μ) and fractions 18C (open triangle, 90.2% lessthan 75μ), 18F (X, 100% less than 30μ), 16F (open diamond, 99% less than20μ), 15F (99% less than 15μ), 14F (solid diamond, 100% less than 10μ)and 13F (solid square, 93% less than 5μ). Fly ash from dry or wet bottomboilers was collected and fractionated into six different sizedistribution fractions as described in the Examples, infra.

FIG. 2 is a graph showing the compressive strength of concrete with age.The concrete samples contain dry bottom boiler fly ash or fractionateddry bottom boiler fly ash as a replacement for 15% (A), 25% (B), 35% (C)and 50% (D) of the cement in the concrete, compared to a standardcontaining cement but no concrete (open squares). Samples contain thefractionated fly ash samples as described in FIG. 1 and the Examples:3FCxx (plus-sign, 3F fly ash fraction, xx stands for the percentage offly ash used to replace cement); 6FCxx (open diamond, 6F fly ashfraction); 10FCxx (open triangle, 10F fly ash fraction); 11FCxx (X, 11Ffly ash fraction); 1CCxx (open inverted triangle, 1C fly ash fraction);and CDRYxx (open square [uniformly of lower compressive strength at eachtime point than the control sample], original dry bottom boiler fly ashfeed).

FIG. 3 presents graphs showing the compressive strength of concrete withage. The concrete contains wet bottom boiler fly ash or fractionated wetbottom boiler fly ash as a replacement for 15% (A), 25% (B), 35% (C),and 50% (D) of cement in the concrete. The fractionated fly ashes are asdescribed in FIG. 1 and the Examples: CCCC (open squares, controlcontaining no fly ash); 13FCxx (plus signs, 13F fly ash fraction, xxstands for the percentage of fly ash used to replace cement); 15FCxx(open diamonds, 15F fly ash fraction); 16FCxx (open triangles, 16F flyash fraction); 18FCxx (X, 18F fly ash fraction); 18CCxx (inverted opentriangle, 18C fly ash fraction); and CWETxx (open square [uniformly oflower compressive strength at each time point than the control sample],original wet bottom boiler fly ash feed).

FIG. 4 presents graphs showing the compressive strength gain over timeof concrete samples in which cement is replaced with 15% (A) or 25% (B)with either silica fume or the finest fraction of fractionated drybottom boiler or wet bottom boiler fly ash. (A) CSF15 (plus sign,replacement with silica fume); C3F15 (open triangle, replacement withdry bottom boiler fraction 3F); C13F15 (inverted open triangle,replacement with wet bottom boiler fraction 13F); and CSF (open square,control containing neither fly ash nor silica fume). (B) CSF25 (opendiamond, replacement with silica fume); C3F25 (X, replacement with drybottom boiler fraction 3F); C13F25 (closed square, replacement with wetbottom boiler fraction 13F); and CSF (open square, control containingneither fly ash nor silica fume).

FIG. 5 presents graphs showing the compressive strength of mortarsamples containing 15% fly ash as a replacement for cement with age. (A)Feed and fractionated dry bottom boiler fly ash as described in FIG. 1and the Examples: CF (open squares, control containing no fly ash); 3Fxx(plus sign, 3F fly ash fraction, in which xx stands for the percentreplacement of cement with fly ash); 5Fxx (open diamond, 5F fly ashfraction); 10Fxx (X, 10F fly ash fraction); 11Fxx (open invertedtriangle, 11F fly ash fraction); 1Cxx (open squares [of much lowercompressive strength than control], 1C fly ash fraction; and DRYxx (plussigns [of lower compressive strength than the 3F-containing samples],feed dry bottom boiler fly ash). (B) Feed and fractionated wet bottomboiler fly ash as described in FIG. 1 and the Examples: CF (opensquares, control containing no fly ash); 13Fxx (plus sign, 13F fly ashfraction, in which xx stands for the percent replacement of cement withfly ash); 14Fxx (open diamond, 14F fly ash fraction); 15Fxx (X, 15F flyash fraction); 18Fxx (open inverted triangle, 18F fly ash fraction);18Cxx (open squares [of much lower compressive strength than control],18C fly ash fraction; and WETxx (plus signs [of lower compressivestrength than the 13F-containing samples], feed wet bottom boiler flyash).

FIG. 6 presents graphs similar to FIG. 5 showing the compressivestrength of mortar samples containing 25% fractionated ornon-fractionated dry bottom boiler fly ash (A) or wet bottom boiler flyash (B) as a replacement for cement, with age. The symbols are the sameas for FIG. 5.

FIG. 7 presents graphs similar to FIGS. 5 and 6 showing the compressivestrength of mortar samples containing 50% fractionated ornon-fractionated dry bottom boiler fly ash (A) or wet bottom boiler flyash (B) as a replacement for cement, with age. The symbols are the sameas for FIG. 5.

FIG. 8 presents graphs showing the relationship between compressivestrength and median fly ash diameter for fractionated dry bottom boilerfly ash concrete. The concrete samples contain 15% (A), 25% (B), 35%(C), and 50% (D) fly ash as a replacement for cement. Compressivestrength was determined at day 1 (open square), day 7 (plus sign), day14 (open diamond), day 28 (open triangle), day 56 (X), day 90 (invertedopen triangle) and day 180 (open square, with much high compressivestrength values than the values at day 1).

FIG. 9 presents graphs showing the relationship between compressivestrength and median fly ash diameter for fractionated wet bottom boilerfly ash concrete. The concrete samples contain 15% (A), 25% (B), 35%(C), and 50% (D) fly ash as a replacement for cement. The symbols arethe same as for FIG. 8.

FIG. 10 is a graph showing the relationship between the a variable B(coefficient of B) in formulae 3, 5 and 6 and cement content of aconcrete mixture. The cement content is expressed as a percentage, byweight, of cementitious materials in the mixture.

FIG. 11 presents graphs showing the predicted (solid line curve) andmeasured (open squares) compressive strength of concrete containing the6F fly ash (dry bottom boiler fly ash) fraction as a replacement for 15%(A), 25% (B), 35% (C), and 50% (D) of cement in the concrete.

FIG. 12 presents graphs showing the predicted (solid line curve) andmeasured (open squares) compressive strength of concrete containing the16F fly ash (wet bottom boiler fly ash) fraction as a replacement for15% (A), 25% (B), 35% (C), and 50% (D) of cement in the concrete.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention relates to hardenable mixturescomprising fly ash of a defined fineness as a replacement for cement incementitious materials, which hardenable mixtures achieve compressivestrength that is about equal to or greater than the compressive strengthof the same hardenable mixture without fly ash. The invention furtherprovides for replacement of a portion of the fine aggregates in ahardenable mixture with fly ash of a defined fineness. The inventionfurther relates to methods for predicting the compressive strength of ahardenable mixture comprising fly ash, based on the degree of finenessof the fly ash. In particular embodiments, the hardenable mixture can beconcrete or mortar, as hereinafter defined.

Throughout this specification, where specific ratios, percentages, orproportions are mentioned, they are determined by weight and not byvolume.

The present invention is based, in part, on the observation thatregardless of the source and chemical composition of fly ash, thepozzolanic properties of the fly ash primarily depend on the degree offineness of the fly ash. It has been surprisingly found thatfractionation of fly ash into fractions of a defined fineness modulus asherein defined provides a high degree of quality control, regardless ofthe classification or combustion conditions of the fly ash.

As used herein, the term "fly ash" refers to a solid material having achemical composition similar to or the same as the composition of thematerial that is produced during the combustion of powdered coal. In aspecific aspect, the solid material is the material remaining after thecombustion of powdered coal. ACI Committee 116 (1990, ACI 116-85, ACIManual of Concrete Practice Part I, American Concrete Institute,Detroit) defines fly ash as "the finely divided residue resulting fromthe combustion of ground or powder coal which is transported form thefirebox through the flue gases", and the term "fly ash" as used hereinencompasses this definition. Generally, fly ash derived from variouscoals have differences in chemical composition, but the principalcomponents of fly ash are SiO₂ (25% to 60%), Al₂ O₃ (10% to 30%), andFe₂ O₃ (5% to 25%). The MgO content of fly ash is generally not greaterthan 5%. Thus, the term fly ash generally refers to solid powderscomprising from about 25% to about 60% silica, from about 10% to about30% Al₂ O₃, from about 5% to about 25% Fe₂ O₃, from about 0% to about20% CaO, and from about 0% to about 5% MgO.

The term "fly ash" further contemplates synthetic fly ash, which may beprepared to have the same performance characteristics as fly ash asdescribed herein.

Presently, fly ash is classified primarily in two groups: Class C andClass F, according to the ASTM C 618 (1990, supra). Class F is generallyproduced by burning anthracite or bituminous coal, and Class C resultsfrom sub-bituminous coal or lignite. Generally, the fly ash from thecombustion of sub-bituminous coals contains more CaO and less Fe₂ O₃than fly ash from bituminous coal (Berry and Malhotra, 1980, ACI J.Proceedings 77:59-73). Thus, the CaO content of the Class C fly ash isusually higher than 10%, with the sum of the oxides of SiO₂, Al₂ O₃ andFe₂ O₃ not less than 50%. For Class F fly ash the CaO content isnormally less than 10% and the sum of the above mentioned oxides is notless than 70%.

The glassy phase of fly ash depends essentially on the combustionconditions and type of boiler. Non-fractionated fly ash obtained fromdifferent boilers, such as dry bottom boilers or wet bottom boilers, hasbeen found to behave differently. Boilers that achieve highertemperature yield fly ash with a more developed or pronounced glassyphase. Alternatively, combustion in the presence of a fluxing agent,which reduces the fusion temperature of the fly ash, can also increasethe glassy phase of fly ash produced by combustion for lower temperatureboilers. Compressive strength of a hardenable mixture containing fly ashmay depend in part on the glassy phase of the fly ash, so generally flyash produced for higher temperature boilers, or produced in the presenceof a fluxing agent, or both, may be preferred. However, as demonstratedherein, the fineness modulus is the most important paramter forcompressive strength, and fractionated fly ash from any source, with adefined fineness modulus, can be used according to the invention.

Although fly ash generally comes in a dry and finely divided form, inmany instances, due to weathering and transportation processes, fly ashbecomes wet and often forms lumps. Such fly ash can be less reactive.

Pozzolan, as defined by ASTM C 593 (1990, ASTM C 593-89, Annual Book ofASTM Standards, Vol. 04.02), is "a siliceous or alumino-siliceousmaterial that in itself possesses little or no cementitious value butthat in finely divided form and in the presence of moisture willchemically react with alkali and alkaline earth hydroxides at ordinarytemperatures to form or assist in forming compounds possessingcementitious properties."

The present invention relates to the determination of the finenessmodulus of fractionated fly ash. As used herein, the term "finenessmodulus" refers to a measure of the distribution of volumes of particlesof fly ash or distribution of particle sizes of the fly ash. Accordingto the present invention, the fineness modulus is a distributionanalysis that is much more informative than an average or medianpartical diameter determination or total surface area determination. Thevalue of fineness modulus corresponds to the fineness of a fraction offly ash, or to non-fractionated fly ash. Thus, a fraction of fly ashcontaining a distribution of particles having smaller size, e.g., amedian diameter that falls within a smaller range set, will have afineness modulus value that is lower than a fraction of fly ashcontaining a distribution of particles having somewhat larger size,e.g., a median diameter that falls within a larger range set, ornon-fractionated fly ash. According to the present invention, lowervalues of fineness modulus are preferred, since hardenable mixtures thatcontain fractions having a lower fineness modulus achieve compressivestrength gains more rapidly. In another embodiment, larger values offineness modulus may be preferred, where a slower rate of compressivestrength gain may be desired.

Thus, the present invention is directed, in part, to use of fractionatedfly ash, in which the fly ash particles in any given fraction have amore uniform distribution of volumes or sizes than non-fractionated flyash.

Preferably, the fineness modulus is determined as the sum of thepercentage of fly ash remaining on each of a series of different sizedsieves. Accordingly, the term "fineness modulus" refers to a relativevalue, which can vary depending on the series of sieves chosen. Since,according to the instant invention, fly ash particles of smaller size ordiameter are preferred for use in hardenable mixtures, more accuratedeterminations of fineness modulus are available if a series of smallersieves are chosen. Preferably, the size of the sieves is predominantlybelow 10μ, e.g., the sieves may be 0.5, 1, 2, 3, 4, 5, 6, 7, 8 and 10microns, with sieves ranging up to 300 microns being useful. The numberof sieves sized 10 microns or less should be at least one more than thenumber of sieves sized greater than 10 microns. In a preferredembodiment, the number of sieves sized 10 microns or less in at leastfive. Although in a specific embodiment, dry seives are used tocalculate a value for the fineness modulus, other methods, such as wetseiving, can also be used.

The greater the number of sieves sized 10 microns or less, the greaterthe absolute value of fineness modulus. Accordingly, where sieves of0.5, 1, 2, 3, 4, 5, 6, 7, 8, and 10 microns are used, the finenessmodulus will be a higher absolute number, reflective of the greaterdegree of accuracy of determination of this value for the smallerdiameter or smaller size fly ash particles.

It has been found that other descriptions of fly ash, such as percentretention on a 45μ (No. 325) sieve, are too crude to provide an accurateand quantitative value for estimating compressive strength gain, or forpreparing a hardenable mixture that has satisfactory compressivestrength. Similarly, it has been found that a measure such as the Blainefineness, which is actually a determination of the average surface areaof fly ash particles somewhat proportional to, but not congruent to sizeor volume, is also not useful for predicting compressive strength gain,or for preparing a hardenable mixture that has satisfactory compressivestrength. In a specific example, infra, compressive strength isindependent of Blaine fineness at a Blaine fineness of greater thanabout 4000 cm² /g, when fractionated fly ash is used to replace 35% ofthe cement, whereas compressive strength varies with median diameterover the entire range of diameters tested.

Although not intending to be bound by any particular theory orhypothesis, it is believed that dissolution of fly ash in a hardenablemixture, whereby the pozzolanic properties of the fly ash can contributeto compressive strength of the hardenable mixture, is acutely dependenton the size distribution of the fly ash to a certain minimum size. Thedata disclosed in the Examples, infra, support a conclusion that the flyash contribution to compressive strength of a hardenable mixture dependson the distribution of particle volumes, or sizes. Above a minimum size,the contribution diminishes. Below this minimum size, strength of theconcrete appears to be independent of size. Most surprising is thediscovery that size, rather than surface area, e.g., as measured asBlaine fineness, is the more critical factor. This observation issurprising because the surface area hypothetically determines thereactivity of a particle, since surface functional groups are presumablymore available for reaction.

The pozzolanic reaction of fly ash in a hardenable mixture comprisingcement is the reaction between constituents of the fly ash and calciumhydroxide. It is generally assumed to take place on the surface of flyash particles, between silicates and aluminates from the glass phase ofthe fly ash and hydroxide ion in the pore solution (Plowman, 1984,Proceedings, 2nd Int'l Conference on Ash Technology and Marketing,London, pp. 437-443). However, the result of the research leading to thepresent invention undicates that thte pozzolanic reactions of the ashare dependent on the folumen of the fly ash aprticles: the smaller theparticle volumen, the more rapidly it completes its reaction with thecement to contribute to compressive strength. The rate of solubility andreactivity of these glassy phases in different types of fly ash dependson the glassy phase of fly ash, which in turn depends on the combustiontemperature of the boiler that produced the fly ash. In addition to theeffect of combustion conditions on the glassy phase of fly ash,different fly ashes from one class can behave differently, depending onthe SiO₂, Al₂ O₃ and Fe₂ O₃ content, and other factors such as theparticle size distribution and storage conditions of the ash (see Aitcinet al, 1986, supra; Liskowitz et al., 1983, supra).

During hydration, portland cement produces a surfeit of lime (CaO) thatis released to the pore spaces. It is the presence of this lime thatallows the reaction between the silica components in fly ash and calciumhydroxide to form additional calcium silicate hydrate [C--S--H]. He etal. (1984, Cement and Concrete Research 14:505-511) showed that thecontent of crystalline calcium hydroxide in the fly ash-portland cementpastes decreases as a result of the addition of fly ash, most likelyresulting from a reaction of calcium with alumina and silica from flyash to form addition C--S--H. This process stabilizes the concrete,reduces permeability and increases resistance to chemical attacks.

Fractionation of fly ash can be accomplished by any means known in theart. Preferably, fractionation proceeds with an air classifying system.In a specific embodiment, infra, a MICRO-SIZER air classifying systemwas used to fractionate fly ash in six different particle size ranges.In another embodiment, the fly ash can be fractionated by sieving. Forexample, a 45μ or smaller sieve can be used to select for particles of adefined maximum size. In a further embodiment, the fly ash can be groundto a desired size or fineness. However, this method is not preferred, asthe grinding process yields non-uniform particles and may introducemetallic or other impurities from the grinder itself.

The term "cement" as used herein refers to a powder comprising alumina,silica, lime, iron oxide and magnesia burned together in a kiln andfinely pulverized, which upon mixing with water binds or unites othermaterials present in the mixture in a hard mixture. Thus, the hardenablemixtures of the invention comprise cement. Generally, the term cementrefers to hydraulic cements such as, but not limited to, portlandcement, in particular portland type I, II, III, IV and V cements.

As used herein, the term "cementitious materials" refers to the portionof a hardenable mixture that provides for binding or uniting the othermaterials present in the mixture, and thus includes cement andpozzolanic fly ash. Fly ash can comprise from about 5% to about 50% ofthe cementitious materials in a hardenable mixture of the invention;preferably, fly ash comprises from about 10% to about 35% ofcementitious materials. The balance of cementitious materials willgenerally be cement, in particular Portland cement. In a specificembodiment, infra, the hardenable mixtures of the invention compriseportland type I cement.

The term "concrete" refers to a hardenable mixture comprisingcementitious materials; a fine aggregate, such as sand: a coarseaggregate, such as but not limited to crushed basalt coarse aggregate;and water. Concrete of the invention further comprises fly ash havingdefined fineness. In a specific embodiment, the fly ash makes up fromabout 10% to about 50% of the cementitious materials. In a furtheraspect, the fly ash is used as fine aggregate in a ratio of from about4:1 to about 1:1 to sand. In yet a further embodiment, the fly ash is anadditive in addition to a replacement of cement, or a replacement ofcement and fine aggregate.

In specific embodiments, concrete of the invention comprises about 1part by weight cementitious materials, about 1 to about 3 parts byweight fine aggregate, about 1 to about 5 parts by weight coarseaggregate, and about 0.35 to about 0.6 parts by weight water, such thatthe ratio of cementitious materials to water ranges from approximately3:1 to 1.5:1; preferably, the ratio of cementitious materials to wateris about 2:1. In a specific embodiment, the concrete comprises 1 partcementitious materials, 2 parts siliceous river sand or Ottawa sand, 3parts 3/8" crushed basalt coarse aggregate, and 0.5 parts water.

The term "mortar" refers to a hardenable mixture comprising cementitiousmaterials; a fine aggregate, such as sand; and water. Mortar of theinvention further comprises fly ash having defined fineness. In aspecific embodiment, the fly ash makes up from about 10% to about 50% ofthe cementitious materials. In a further aspect, the fly ash is used asfine aggregate in a ratio of from about 4:1 to about 1:1 to sand. In yeta further embodiment, the fly ash is an additive in addition to areplacement of cement, or a replacement of cement and fine aggregate.

In specific embodiments, mortar of the invention comprises about 1 partby weight cementitious materials, about 1 to about 3 parts by weightfine aggregate, and about 0.5 parts by weight water, such that the ratioof cementitious materials to water is approximately 2:1. In a specificembodiment, the mortar comprises 1 part cementitious materials, 2.75parts Ottawa sand, and 0.5 parts water.

As noted above, fly ash can be used as a fine aggregate in concrete ormortar, in addition to having a role as a cementitious material. It hasbeen found that substituting fly ash for a conventional fine aggregate,such as sand, provides the advantages of increased compressive strengthof the concrete or mortar since the total amount of fly ash in thehardenable composition is the same, with a rapid rate of increase ofcompressive strength because the amount of cement in the cementitiousmaterials is greater.

According to the present invention, the hardenable mixture can furthercomprise one or more of the following: kiln dust, e.g., the dustgenerated in the manufacture of cement; silica fume, which is aby-product from the silicon metal industry usually consisting of about96%-98% reactive SiO₂, and which generally comes in very fine particlesizes of less than 1 micron; superplasticizer, such as Daracem-100 (W.R. Grace),an expensive but common additive for concrete used to decreasethe water requirement for mixing the concrete; and a dispersing agent,such as sodium hexametaphosphate (NaPO₃). The use of a dispersing agentis particularly preferred when weathered fly ash is incorporated in thehardenable mixture.

Addition of silica fume can enhance the early rate of strength gain of ahardenable mixture, and therefore may be a desirable component ofhardenable mixtures of the invention.

In a specific embodiment, a hardenable mixture of the invention may alsocontain glass fibers for reinforcement. The use of glass fibers inhardenable mixtures of the invention for reinforcement can be achievedbecause the fly ash, particularly finer fractions of fly ash, reactsmore readily than glass fibers with reactive components of the cement,e.g., Ca(OH)₂, thus preventing long term reaction of the glass fiberswith these reactive components, which would otherwise degrade the glassfibers. The most inert hardenable mixtures result are those that containapproximately equal amounts of fly ash, or fly ash and silica fume (asdiscussed below), and cement. The ability of fly ash to neutralizereactive agents in cement is discussed in greater detail in U.S.application Ser. No. 08/246,861, attorney docket No. 715-1-036, filedMay 20, 1994, entitled "SULFATE AND ACID RESISTANT CONCRETE AND MORTAR"by the instant inventors.

In another specific embodiment, a hardenable mixture of the inventionfurther comprises glass fibers, and silica fume. Silica fume reacts morereadily with reactive components of cement than the glass fibers, andthus can provide early desirable protection of the glass fibers fromdegradation as well as early compressive strength gains. Subsequently,the fly ash will react with such reactive components, thus precludingearly and late reactivity of glass fibers. As noted above, reaction ofglass fibers with alkali and alkali earth compounds can lead todegradation of the glass fibers, and loss of tensile strength of thehardenable mixture.

Concrete beams of the invention with dimensions of 3"×6"×27" can be usedto evaluate the bending strength of fly ash concrete, e.g., using simplebeam with third-point loading. Preferably, such test procedures are inaccordance with ASTM C 78 (1990, ASTM C 78-84, Annual Book of ASTMStandards, Vol 04.02).

The present invention will be better understood by reference to thefollowing Examples, which are provided by way of exemplification and notby way of limitation.

EXAMPLES

Fly ashes used in this study were collected from a utility in theNortheastern section of the U.S. Fly ashes of different sources namedDH, H, M, and P were used in this program. The last sample was obtainedin both dry and weathered states as described earlier.

The standard ASTM 2"×2"×2" cube and 3"×6" cylinder specimens forstudying the compressive strength of mortar and concrete, respectively,were used. The 3"×6"×27" beam specimens were selected for studying thebending or flexural strength of concrete. All tests were performed on aMTS closed-loop servo hydraulic testing machine.

Materials

Materials used in this study consisted of standard portland cement typeI, Ottawa sand, siliceous sand (river sand), coarse aggregate, fly ash,kiln dust, silica fume, superplasticizer, dispersing agent, and water.

Two kinds of sand were used. Graded sand predominantly graded betweenthe No. 300 (0.06 mm) sieve and the No. 100 (0.150 mm) sieve conformingto ASTM C-778 (1990, "Specification for Standard Sand," Annual Book ofASTM Standards, Vol. 04.08) was used as a standard sand. Another localsiliceous sand (river sand) passing through sieve No. 4 (opening size4.75 mm) was also used for casting mortar and concrete.

Crushed basalt coarse aggregate size of 3/8 was used for castingconcrete.

Wet bottom boiler and dry bottom boiler fly ashes were selected for thestudy. These two type of fly ashes were further fractionated intodifferent particle sizes for additional study.

Silica fume (produced in the manufacture of microelectronic chips) ofvery fine particle of size less than 1 micron and 96-98% reactive SiO2was used in powder form. The addition of silica fume was intended toproduce high strength concrete.

Superplasticizer (Daracem-100, W. R. Grace) was used according tostandard procedures.

Sodium hexametaphosphate (NaPO₃) was normally used as a dispersingagent. The addition of dispersing agent in the fly ash concrete mix wasto ensure the lumps of weathered fly ash were dispersed into fineparticles and could as a result, be more reactive.

Tap water was used throughout.

The chemical composition of fly ashes and cement were determined byX-Ray Fluorescence (ASTM D-4326 1990, "Test Method for Major and MinorElements in Coal and Coke Ash by X-Ray Fluorescence," Annual Book ofASTM Standards, Vol. 05.05).

Fly Ash Fineness

The fineness of fly ash was measured using two different standardmethods; the Blaine air permeability and the fineness by the 45 microns(No. 325 sieve). Fineness was also determined as the fineness modulus,as described.

For the Blaine air permeability (Blaine fineness), the fineness wasexpressed in terms of the specific surface, expressed as total surfacearea in square centimeters per gram, or square meters per kilogram, offly ash. The result obtained from the Blaine method was a measure ofrelative fineness rather than absolute fineness. The test procedurefollowed ASTM C 204 (1990, "Test Method for Fineness of PortlandCement," ASTM C 204-89, Annual Book of ASTM Standards, Vol. 04.01).

The fineness of fly ash retained on the sieve 45 microns (No. 325 sieve)was determined by the amount of fly ash retained when wet sieved on theNo. 325 sieve in accordance with the ASTM C 430 (1990, "Test Method forFineness of Hydraulic Cement by the 45-Micron (No. 325) Sieve," ASTM C430-89, Annual Book of ASTM Standards, Vol. 04.01) test method forhydraulic cement.

Fineness modulus was determined by the summation of the percentage offly ash that retained on the following sieve sizes: 0, 1, 1.5, 2, 3, 5,10, 20, 45, 75, 150, and 300 microns.

The setting time of concrete or mortar mixtures was determined by Vicatneedle and Gillmore needle tests. The test methods followed ASTM C-191(1990, "Test Method for Setting Time of Hydraulic Cement by VicatNeedle," ASTM C 191-82, Annual Book of ASTM Standards, Vol. 04.01) forthe Vicar test and ASTM C 266 (1990, "Test Method for Setting ofHydraulic Cement Paste by Gillmore Needles," ASTM C 266-89, Annual Bookof ASTM Standards, Vol. 04.01) for the Gillmore test.

Fly Ash Mortar

DH, H, dry, and weathered fly ashes were mixed with cement and Ottawasand. The replacement of a portion of portland cement by fly ash variedas 0%, 15%, 25% and 35% by weight of cementitious (cement+fly ash)materials. The specimens were mixed and cast in accordance with ASTM C109 (1990, "Test Method for Compressive Strength of Hydraulic CementMortars . . . ," ASTM C 109-88, Annual Book of ASTM Standards, Vol.04.01). All specimens were cured in saturated lime water and tested atthe age of 1, 3, 7, 14, 28, 56, and 90 days.

Fly Ash as a Replacement

Fly ashes were used as replacement of cement. By keeping the water,river sand, and cementitious (cement+fly ash) materials as constants,cement was replaced by fly ash. The replacement of fly ash was variedfrom 15% to 50% by weight of cementitious materials. All the specimenswere cured in saturated lime water until the time of testing. This wasto ensure that moisture and lime are available to provide any potentialreaction which may occur. The compressive strengths of 2"×2"×2" cubemortars were tested at 1, 3, 7, 14, 28, 56, 90 and 180 days.

Fly Ash as an Additive

Fly ashes were used as an additive in mortar. In some instances, 10% ofsand was replaced by fly ash. By keeping the cement, river sand, andwater as constants, fly ash was added directly in the mix. The additionof fly ash was varied from 15% to 50% by weight of cement. All thespecimens were cured in saturated lime water and tested for theircompressive strengths at 1, 3, 7, 14, 28, 56, 90 and 180 days.

Fractionated Fly Ash Concrete and Mortar

Dry and wet bottom boiler fly ashes were separated into differentparticle sizes by using the Micro-Sizer Air Classifying System. The flyash was fractionated into six particle size distributions. Thefractionated fly ashes and the original feed fly ashes were used toreplace 15%, 25% 35% and 50% of cement by weight of cementitiousmaterials. The compressive strengths of fractionated fly ash concretewere tested from 1 day to 180 days. The effect of particle size from0-5, 0-10, 0-15, 0-20, 0-30, 0-44 microns, and the original feed flyashes, were investigated and compared with the control concrete. The3"×6" cylinder was used to determine the compressive strength offractionated fly ash concrete. The standard size of 2"×2"×2" cube wasused to determine the compressive strength of fractionated fly ashmortars. The mix proportion of fractionated fly ash mortar is shown inTable 1.

                  TABLE 1                                                         ______________________________________                                        Mix Proportion of Fractionated Fly Ash Mortar                                               Fractionated Fly Ash (Dry and                                                 Wet Bottom Boiler) By Weight                                    Ingredients     0      15%      25%  50%                                      ______________________________________                                        Cement          1.00   0.85     0.75 0.50                                     Fly Ash         --     0.15     0.25 0.50                                     Sand            2.75   2.75     2.75 2.75                                     Water           0.50   0.50     0.50 0.50                                     Water/(Cem + FA)                                                                              0.50   0.50     0.50 0.50                                     ______________________________________                                    

High Strength Fly Ash and Silica Fume Concrete

The very fine particle sizes of fly ashes, i.e., the particles smallerthan 5 microns, were employed to produce higher strength fly ashconcrete. Fifteen and twenty five percent of fly ash by weight ofcementitious materials were used in the concrete as a replacement forcement. Silica fume in the powder form was also used in the sameproportion as the fly ash. The compressive strength of the high strengthfly ash concrete and silica fume concrete were determined and compared.The mix proportion of high strength fly ash and silica fume concrete isshown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Mix Proportion of High Strength Fly Ash                                       and Silica Fume Concrete                                                                    CSF, Control                                                                             15% Repl. 25% Repl.                                  Ingredient    (lb)       (lb)      (lb)                                       ______________________________________                                        Cement        10         8.5       7.5                                        Fly Ash or Silica Fume                                                                      --         1.5       2.5                                        River Sand    20         20        20                                         Aggregate, Basalt 3/8"                                                                      30         30        30                                         Super P.      100 ml     100 ml    100 ml                                     Water         4.17       4.17      4.17                                       Water/(Cementitious)                                                                        0.417      0.417     0.417                                      ______________________________________                                    

Chemical Composition of Fractionated Fly Ashes

The chemical composition of fractionated fly ashes are shown in Table 3.Sample CEM is the cement sample used in this study. Samples DRY and WETare the fly ashes from the original feed of dry and wet bottom boilerashes, respectively. 3F is the finest fly ash sample of the dry bottomboiler ash and 13F is the finest sample of the wet bottom boiler ash.The coarsest fly ashes samples of dry and wet bottom boiler ash are 1Cand 18C, respectively.

Both wet and dry bottom boiler fly ashes used herein were classified asClass F fly ash according to ASTM C-618 (1990, supra). Most of thefractionated fly ashes varied slightly in the oxide composition withchanges in particle size. It has been reported that separation of ClassF (high calcium) fly ash into size fractions does not result insignificant chemical, morphological or mineralogical specificationbetween particles (Hemming and Berry, 1986, Symposium Proceedings, FlyAsh and Coal Conversion By-Products: Characterization, Utilization andDisposal II, Material Research Society 65:91-130). The SiO₂ contenttends to be lower when the particle size is larger. Differences inchemical compositions of the two fly ashes were observed in the SiO₂,Fe₂ O₃, and CaO contents. Samples of the dry bottom boiler fly ash wereabout 10% richer in SiO₂ than the wet bottom boiler fly ash. The CaOcontent of the dry bottom boiler fly ash varied from 1.90% to 2.99%,while for wet bottom boiler fly ash, the CaO varied from 6.55% to 7.38%.Fe₂ O₃ content of wet bottom boiler fly ash was about twice as high inwet bottom boiler than dry bottom boiler fly ash. The highestconcentration of Fe₂ O₃ of each type of fly ashes was observed in thecoarsest particle sizes, i.e., 1C and 18C. Chemical composition of thefly ashes is shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________    Chemical Composition of Fractionated                                          Fly Ashes and Cement                                                          Chemical Composition (%)                                                      Sam LOI SO.sub.3                                                                         SiO.sub.2                                                                         A1.sub.2 O.sub.3                                                                  Fe.sub.2 O.sub.3                                                                   CaO                                                                              K.sub.2 O                                                                         MgO                                                                              Na.sub.2 O                                  __________________________________________________________________________    CEM 0.73                                                                              2.53                                                                             20.07                                                                             8.84                                                                              1.41 60.14                                                                            0.86                                                                              2.49                                                                             0.28                                        3F0 4.97                                                                              1.69                                                                             49.89                                                                             26.94                                                                             5.43 2.99                                                                             1.76                                                                              0.99                                                                             0.33                                        5F  4.10                                                                              1.53                                                                             50.27                                                                             26.74                                                                             5.30 2.95                                                                             1.74                                                                              0.93                                                                             0.33                                        6F  3.12                                                                              1.09                                                                             51.40                                                                             26.54                                                                             4.91 2.72                                                                             1.71                                                                              0.74                                                                             0.31                                        10F 2.52                                                                              0.72                                                                             51.98                                                                             26.23                                                                             4.44 2.28                                                                             1.60                                                                              0.54                                                                             0.29                                        11F 2.04                                                                              0.53                                                                             51.27                                                                             26.28                                                                             4.42 2.02                                                                             1.55                                                                              0.49                                                                             0.26                                        1C  1.46                                                                              0.39                                                                             53.01                                                                             26.50                                                                             5.66 1.90                                                                             1.61                                                                              0.56                                                                             0.24                                        DRY 2.75                                                                              0.98                                                                             52.25                                                                             26.72                                                                             5.43 2.41                                                                             1.67                                                                              0.69                                                                             0.28                                        13F 2.67                                                                              3.81                                                                             38.93                                                                             24.91                                                                             12.89                                                                              6.85                                                                             2.10                                                                              1.55                                                                             1.31                                        14F 1.94                                                                              3.47                                                                             39.72                                                                             25.08                                                                             13.02                                                                              6.71                                                                             2.11                                                                              1.50                                                                             1.31                                        15F 1.88                                                                              3.33                                                                             40.25                                                                             25.02                                                                             13.12                                                                              6.60                                                                             2.11                                                                              1.47                                                                             1.30                                        16F 2.06                                                                              3.05                                                                             40.65                                                                             24.92                                                                             13.26                                                                              6.55                                                                             2.09                                                                              1.41                                                                             1.26                                        18F 1.94                                                                              2.94                                                                             41.56                                                                             24.47                                                                             14.21                                                                              6.58                                                                             2.01                                                                              1.40                                                                             1.17                                        18C 2.55                                                                              2.40                                                                             43.25                                                                             23.31                                                                             17.19                                                                              7.38                                                                             2.00                                                                              1.30                                                                             0.88                                        WET 2.05                                                                              3.13                                                                             41.54                                                                             24.74                                                                             14.83                                                                              6.89                                                                             2.07                                                                              1.43                                                                             1.17                                        __________________________________________________________________________

It is interesting to note that after fly ash was fractionated intodifferent sizes, loss of ignition (LOI) of the finest particle washigher than for larger particles. In other words, the LOI contentgradually decreased as the particle size increased. Ravina (1980, Cementand Concrete Research 10:573-80) also reported that the finest particleof fly ashes has the highest LOI values. Ukita et al. (1989, Fly Ash,Silica Fume, Slag, and Natural Pozzolans In Concrete, SP-114, AmericanConcrete Institute, Detroit, pp. 219-40) also showed that althoughchemical composition did not change when the median diameter of fly ashdecreased from 17.6 microns to 3.3 microns, LOI increased from 2.78 to4.37.

Our observations and these prior reports conflict with the report of ACICommittee 226 (1987, "Use of Fly Ash In Concrete," ACI 226.3R-87, ACI J.Proceedings 84:381-409)and of Sheu et al. (1990, Symposium Proceedings,Fly Ash and Coal Conversion By-Products: Characterization, Utilizationand Disposal VI, Materials Research Society 178:159-166), which statethat the coarse fraction of fly ash usually has a higher LOI than thefine fraction.

Particle Size Analysis of Fractionated Fly Ashes

The particle size distributions of Fractionated fly ashes from the dryand wet bottom boilers are shown in FIGS. 1A and 1B, respectively. Thecurves for the original feed fly ashes are not as steep as others sincethe non-fractionated original feed ash includes the entire range ofsizes, and thus a wider range of size distributions than fractionatedsamples.

The percentage of fly ash in each fraction having a size less than aparticular size is indicated in parentheses in each curve. For example,in case of the 3F fly ash, the finest of dry bottom boiler fly ash, 3F(90%-5 μm) means that 90% of the fly ash particles are smaller than 5microns.

From the original feed, each type of fly ash was fractionated into sixranges. As shown in FIGS. 1A and 1B, the particle size of fly ash variedfrom 0-5.5 micron to 0-600 microns. The median diameter of the particlesin each fraction was determined from the curves in FIGS. 1A and 1B byextrapolating from the 50% percent finer value. The median diameters of3F and 13F were 2.11 and 1.84 microns, respectively, while the mediandiameters of the coarsest particle size, 1C and 18C, were 39.45 and29.23 microns, respectively. For wet bottom boiler fly ash, 13F was thefinest fraction and 18C was the coarsest.

The original feed of wet bottom boiler fly ash was found to be finerthan the original feed of dry bottom boiler fly ash. The particle sizesof original feed of dry bottom boiler fly ash varied from about 1 micronto 600 microns, with a median particle size of 13.73 microns. Theoriginal feed of wet bottom boiler fly ash included particles up to 300microns with a median diameter of 6.41 microns. Particles from thesmaller size fractions tend to have a more spherical shapes (Hemming andBerry, 1986, supra).

Fineness of Fractionated Fly Ash

Traditional values of fineness of fly ashes were determined both by wetsieve analysis and by the Blaine fineness together with the specificgravity of fly ashes, which are shown in Table 4. Median diameter, thediameter of which 50 percent of particles are larger than this size, isalso presented in this table. According to ASTM C-618 (1990, supra),specifications, fractionated 1C fly ash is unacceptable for use inconcrete since the percentage of the fly ash retained on sieve No. 325is higher than 34%.

                  TABLE 4                                                         ______________________________________                                        Fineness of Cement and Fractionated Fly Ashes                                            Specific Fineness: Fineness:                                                                            Median                                              Gravity  Retained 45                                                                             Blaine Diameter                                 Sample No. (g/cm.sup.3)                                                                           μm (%) (cm.sup.2 /g)                                                                        (μm)                                  ______________________________________                                        CEM        3.12     --        3815   --                                       3F         2.54     0         7844   2.11                                     5F         2.53     0         6919   2.66                                     6F         2.49     0         4478   5.66                                     10F        2.42     0         2028   12.12                                    11F        2.40     1.0       1744   15.69                                    1C         2.28     42.0      1079   39.45                                    DRY        2.34     20.0      3235   13.73                                    13F        2.75     0         11241  1.84                                     14F        2.73     0         9106   2.50                                     15F        2.64     0         7471   3.09                                     16F        2.61     0         5171   5.54                                     18F        2.51     0         3216   9.84                                     18C        2.42     29.0      1760   29.25                                    WET        2.50     10.0      5017   6.41                                     DRY FA     2.25     22.0      3380   11.51                                    WEATHERED  2.20     18.0      2252   13.22                                    H          2.30     15.0      2748   13.15                                    DH         2.24     26.0      2555   18.30                                    ______________________________________                                    

Two methods were used to measure the fineness of fractionated fly ashes.The first method involved determining the residue on a 45 micron (No.325) sieve. Using the sieve No. 325 method, the fractionated fly ashsamples 3F, 5F, 6F, 10F, 13F, 14F, 15F, 16F and 18F had the samefineness; all of them have zero retention.

The second method was the surface area measurement by air permeabilitytest.

Opinions differ as to whether sieve residue or surface area are betterindicator of fly ash fineness (Cabrera, et al., 1986, Fly Ash, SilicaFume, Slag and Natural Pozzolans in Concrete, SP-91, American ConcreteInstitute, Detroit, pp. 115-144). In the United States, the fineness offly ash is specified by the residue on the 45 micron sieve only. Ravina(1980, Cement and Concrete Research 10:573-580) found that pozzolanicactivity correlates more closely with specific surface areameasurements. In contrast, Lane and Best (1982, Concrete Int'l: Design &Construction 4:81-92) urges that the residue on a 45 micron sieve is amore consistent indicator of pozzolanic activity. White and Roy (1986,Symposium Proceedings, Fly Ash and Coal Conversion By-Products:Characterization, Utilization and Disposal II, Material Research Society65:243-253) also concluded that the fineness parameter given in theBlaine fineness is not as important as the fly ash size fraction lessthan 45 microns.

The results of the present work rebut the conclusions advanced in theWhite and Roy article, especially in the case of fractionated fly ashes,since the results disclosed herein demonstrate that the preferred activeparticle size of fly ash is significantly smaller than 45 microns.

It can be noted from Table 5 that the finer the particle size offractionated fly ashes was, the higher the specific gravity and theBlaine fineness. In general, fly ash of greater fineness had greaterspecific gravity, in agreement with previous investigation (Hansson,1989, Symposium Proceedings, Fly Ash and Coal Conversion By-Products:Characterization, Utilization and Disposal V, Material Research Society136:175-183).

Density of fly ash from different electric generating plants varies from1.97 to 2.89 g/cm³ but normally ranges between about 2.2 to 2.7 g/cm³(Lane and Best, 1982, supra). Work done by McLaren and Digiolin (1990,Coal Combustion and By-Product Utilization Seminar, Pittsburgh, p. 15)reported that Class F fly ash had a mean specific gravity value of 2.40.The specific gravity of fractionated fly ashes varies from 2.28 for thecoarsest fly ash to 2.54 for the finest fly ash for dry bottom boilerfly ash, and from 2.22 for the coarsest to 2.75 for the finest wetbottom boiler fly ash.

The differences in density between dry bottom boiler and wet bottomboiler fly ashes suggest that the very fine particles of wet bottomboiler fly ash are thick-walled, void free, or composed of more denseglasses and crystalline components than dry bottom boiler fly ash(Hemming and Berry, 1986, Symposium Proceedings, Fly Ash and CoalConversion By-Products: Characterization, Utilization and Disposal II,Material Research Society 65:91-103).

The Examples presented herein disclose results of incorporation of flyash of defined particle size distributions in concrete and mortar. Thefractionated fly ashes each have a smaller size range than the originalfeed fly ash, that is, fly ash as received from a storage silo. Due toits narrower range of particle size distribution of fractionated fly ashcompared to the wider range of particle size distributions of theoriginal ash, each fractionated fly ash has a more defined pozzolanicactivity than the original feed fly ash.

In the Examples, Sample CCCC is generally the control sample, i.e.,sample without any fly ash. CDRY and CWET are the samples for concretemixed with the original feed of dry and wet bottom boiler fly ashes,respectively. The fractionated fly ashes used in the sample aredesignated by the number(s) followed by the character. The last twodigits indicate the proportion by weight of fly ash as cementitiousmaterials in the mix. For example, sample "3FC15" means that theconcrete sample consists of 3F fly ash present at 15% by weight ofcementitious materials. Similarly, sample "3FC25" stands for theconcrete sample using 25% of 3F fly ash by weight of cementitiousmaterials.

Example 1

Effect of Fractionated Dry Bottom Boiler Fly Ash on the Strength ofConcrete

The relationship between compressive strength of the fractionated drybottom boiler fly ash concrete and its corresponding age is shown inFIG. 2A, 2B, 2C, and 2D.

The compressive strength of the fractionated dry bottom boiler fly ashconcrete, in which fly ash replaces 15% of cement, relative to control(shown as a percentage) is summarized in Table 5 and FIG. 2A.

                                      TABLE 5                                     __________________________________________________________________________    Percentage Compressive Strength of the                                        Fractionated of Dry Bottom boiler Fly Ash Concrete Over Time                  (15% Replacement)                                                             Sample                                                                             Percentage Compressive Strength of Control                               No.  1-d 7-d  14-d 28-d 56-d 90-d 180-d                                       __________________________________________________________________________    CCCC*                                                                              2157                                                                              6237 7141 8157 8707 9195 10161                                       3FC15                                                                              79.8                                                                              95.3 100.7                                                                              102.0                                                                              104.8                                                                              107.5                                                                              109.2                                       6FC15                                                                              79.6                                                                              92.1 96.4 97.4 102.8                                                                              103.1                                                                              104.4                                       10FC15                                                                             77.6                                                                              90.9 90.7 92.4 96.6 98.0 101.8                                       11FC15                                                                             77.3                                                                              89.0 90.0 90.1 93.5 94.9 96.9                                        1CC15                                                                              74.1                                                                              86.8 89.8 85.5 90.6 89.8 91.2                                        CDRY15                                                                             75.2                                                                              88.6 90.7 91.2 95.3 97.3 99.2                                        __________________________________________________________________________     *Values for control are the actual compressive strength in psi. These are     the 100% values at each time point.                                      

The strength of fractionated fly ash concrete was always lower than thecontrol mix at day 1. Replacement of a portion of cement with Class Ffly ash generally produces lower strength because fly ash acts as arelatively inert component during the early period of hydration (Caretteand Malhortra, 1983, Fly Ash, Silica Fume, Slag, and Other MineralBy-Products in Concrete, SP-79, American Concrete Institute, Detroit,pp. 765-784). This result has also been reported by Plowman (1984,Proceedings, 2nd Int'l Conference on Ash Technology and Marketing,London, pp. 437-443) and Langley et al. (1989, ACI J. Proceedings86:507-514).

With 15% replacement of cement by fractionated fly ashes, thecompressive strength at 1 day was reduced about 20% to 25% compared tothe control (sample CCCC). Variation of the strength correlates with thedifferent particle sizes of fly ash. The finer particle fly ash mediatesa better packing effect than the coarser one, so the rate of strengthgain is greater.

After 14 days of curing, 3FC15 concrete (15% replacement of 3F fly ash)had a compressive strength essentially equal to the control. This meansthat the pozzolanic activity of the finest particle size fly ashproduced greater strength than that achieved by the hydration of cementalone. This increased rate of strength gain result continued, resultingin larger differences between the 3FC15 fly ash concrete and the controlconcrete with time.

With time, larger size fractions also achieved strengths comparable toor greater than control. For example, sample 6FC15 gained the samestrength as the control before the age of 56 days. After about 180 daysof curing, the samples 10FC15 and CDRY15 (15% replacement of theoriginal feed of dry bottom boiler fly ash) achieved the same strengthas the control.

With the coarsest particle size of fly ash in concrete, 1CC15, thecompressive strength varied from 1598 psi at 1 day to 9269 psi at 180days, or from 74.1% to 91.2% relative to the control concrete. Thecompressive strength of sample 3FC15 varies from 1721 psi at 1 day to11100 psi at 180 days, or from 79.8% to 109.2% compared with the controlstrength. Since all the chemical composition of these fractionated flyashes are almost the same, the particle size of fly ash is the majorfactor affecting the compressive strength of fly ash concrete.

The results of compressive strength gain of concrete in which 25% ofcement in cementitious materials is replaced with fractionated drybottom boiler fly ash is shown in Table 6 and FIG. 2B.

                                      TABLE 6                                     __________________________________________________________________________    Percentage Compressive Strength of the                                        Fractionated of Dry Bottom boiler Fly Ash Concrete Over Time                  (25% Replacement)                                                             Sample                                                                             Percentage Compressive Strength (%)                                      No.  1-d 7-d  14-d 28-d 56-d 90-d 180-d                                       __________________________________________________________________________    CCCC 2157                                                                              6237 7141 8157 8707 9195 10161                                       3FC25                                                                              70.0                                                                              84.7 90.9 94.2 98.4 103.3                                                                              105.6                                       6FC25                                                                              68.8                                                                              77.2 81.8 86.5 93.3 95.5 98.2                                        10FC25                                                                             67.1                                                                              75.9 78.7 82.0 88.7 91.0 91.7                                        11FC25                                                                             64.4                                                                              74.3 77.9 80.7 84.9 88.2 89.6                                        1CC25                                                                              63.5                                                                              72.8 75.6 78.0 80.4 81.8 82.2                                        CDRY25                                                                             64.4                                                                              73.6 76.9 80.9 84.9 87.5 89.3                                        __________________________________________________________________________     *Values for control are actual compressive strength in psi, which             constitute 100% at each time point.                                      

When 25% of cement is replaced with the fractionated dry bottom boilerfly ash, early strengths of the concrete are lower than with a 15%replacement with the same fly ash fraction. The results indicate thatthe finer fly ash particles yield greater strength gains than thecoarser particles.

The results of compressive strength gain of concrete in which 35% ofcement in cementitious materials is replaced with fractionated drybottom boiler fly ash is shown in Table 7 and FIG. 2C.

                                      TABLE 7                                     __________________________________________________________________________    Percentage Compressive Strength of the                                        Fractionated of Dry Bottom Boiler Fly Ash Concrete Over Time                  (35% Replacement)                                                             Sample                                                                             Percentage Compressive Strength (%)                                      No.  1-d 7-d  14-d 28-d 56-d 90-d 180-d                                       __________________________________________________________________________    CCCC*                                                                              2157                                                                              6237 7141 8157 8707 9195 10161                                       3FC35                                                                              52.7                                                                              73.8 77.5 80.9 85.9 91.4 99.2                                        6FC35                                                                              45.8                                                                              67.7 74.6 78.2 83.2 87.0 93.0                                        10FC35                                                                             41.2                                                                              62.7 67.7 70.7 74.3 77.6 82.7                                        11FC35                                                                             40.9                                                                              59.9 66.8 68.8 71.4 74.6 79.0                                        1CC35                                                                              39.9                                                                              57.2 63.0 64.2 65.4 67.4 71.3                                        CDRY35                                                                             42.0                                                                              62.4 65.8 71.1 74.0 78.2 82.6                                        __________________________________________________________________________     *Values for control are actual compressive strength in psi, which             constitute 100% at each time point.                                      

With the replacement of fly ash up to 35% by weight of cementitiousmaterials, the compressive strength for fractionated fly ash concrete at1 day varied from 39.9% to 52.7% of the control strength, depending onthe fineness of the fly ash. In general, the compressive strength of thefiner particle mixes was higher than that for the coarser ones.

After 180 days of curing, the compressive strength of fly ash concretemade with 35% original feed of dry bottom boiler fly ash was 8389 psi,or 82.6% of the control concrete. With the finest particle size of flyash, 3F, it took about 180 days for the fly ash concrete to have thesame strength as the control. The compressive strength of 3FC35 variesfrom 1136 psi at 1 day to 10080 psi at 180 days. That is an increase ofabout 8.8 times from 1 day to 180 days. The strength of the coarsestsample, 1C35, at 180 days is only 71.3% of the control strength.

FIG. 2C is a graph showing the relationship of compressive strength toage of concrete samples in which 35% of cementitious materials arefractionated or non-fractionated fly ash, as well as control (no flyash). A number of points are made by this graph. The first is thatinitial rate of strength gain depends critically on the particle sizerange of the fly ash. After this period, which ranges up to about 14 toabout 28 days, the slopes of compressive strength over time (rate ofstrength gain) become parallel, i.e., independent of particle size. Atthese points, rate of strength gain appears to be a diffusion-controlledpozzolanic effect. Nevertheless, in order to achieve acceptablecompressive strength at early time points, which is important inbuilding construction, clearly small particle fly ash fractions arepreferable.

With 50% fly ash of the cementitious materials, all strengths offractionated fly ash concrete are lower than the control strength (FIG.2D). The compressive strength at 1 day varies from 407 psi to 567 psi(from the coarse to the fine particle size of fly ash) or 18.9% to 26.3%of the control strength. This strength is much lower than the controlstrength which is 2157 psi. Of note even with the 50% replacementsample, the compressive strength of fly ash concrete gradually increaseswith time due to the pozzolanic activity of fly ash. The strength of3FC50 varies from 567 psi at 1 day to 8639 psi at 180 days, or 26.3% to85.0% relative to the control.

Also, after 28 days the slope of compressive strength over time of 3FC50concrete is higher than the slope of the control. This means that after28 days the pozzolanic activity of the fly ash contributes more strengththan the strength produced by the hydration of cement.

Example 2

Compressive Strength of Fractionated Wet Bottom Boiler Fly Ash Concrete

The relationship between compressive strength of the fractionated wetbottom boiler fly ash concrete and its corresponding age is shown inFIG. 3A, 3B, 3C and 3D.

The compressive strength of the original feed of wet bottom boiler flyash was higher than that from the dry bottom boiler fly ash at the sameage and for the same mix proportions. This was probably due to the finerparticle size of the wet bottom boiler fly ash.

With 15% replacement of cement by fly ash (FIG. 3A), all the earlystrengths of fractionated fly ash concrete were lower than the control.At 14 days, the compressive strength of 13FC15 was a little higher thanthe control strength. After 56 days, sample 15FC15 gave the samestrength as the control concrete. Samples 16FC15 and 18FC15 achieved thesame strength as the control concrete after 90 days. After 180 days, allof the fractionated fly ash concretes had higher strength than thecontrol concrete except sample 18CC15, which had 95.3% of the controlstrength.

Sample 18CC15 was made up with 18CC ash, which has the residue retainedon sieve No. 325 (45 microns) of 29%. This value is lower than the limitset by ASTM C 618 (1990, supra) of 34%. The 29% retention valueindicates that the active particle size of fly ash in the 18CC fractionis smaller than the 45 micron of sieve opening.

It took 180 days for the original feed of wet bottom boiler fly ashconcrete to gain strength of the same order as the control concrete.These data are summarized in Table 8.

                                      TABLE 8                                     __________________________________________________________________________    Percentage Compressive Strength of the                                        Fractionated Dry Bottom Boiler Fly Ash Concrete Over Time                     (15% Replacement)                                                             Sample                                                                             Percentage Compressive Strength (%)                                      No.  1-d 7-d  14-d 28-d 56-d 90-d 180-d                                       __________________________________________________________________________    CCCC 2157                                                                              6237 7141 8157 8707 9195 10161                                       13FC15                                                                             92.6                                                                              96.4 101.1                                                                              102.1                                                                              106.0                                                                              107.3                                                                              110.2                                       15FC15                                                                             91.7                                                                              94.7 97.2 97.1 101.1                                                                              102.6                                                                              106.4                                       16FC15                                                                             88.0                                                                              92.0 93.0 93.8 98.9 101.4                                                                              105.5                                       18FC15                                                                             85.7                                                                              92.0 92.1 92.0 94.2 99.3 103.4                                       18CC15                                                                             84.4                                                                              87.6 88.0 88.6 89.9 91.2 95.3                                        CWET15                                                                             85.5                                                                              87.7 88.9 90.0 93.1 96.7 100.0                                       __________________________________________________________________________

The results with 25% replacement were close to those observed with 15%replacement with the fractionated wet bottom boiler fly ash concrete,except that the compressive strengths were uniformly lower (FIG. 3B).Early strengths of fractionated fly ash concrete were lower than controlconcrete up to 14 days. At 28 days and longer, sample 13FC25demonstrated higher strength than control strength. At 180 days, thecompressive strength of 13FC25 was 11162 psi, or 109.9% of the controlvalue. Sample 15FC25 reached the same strength as the control concretebefore 56 days. Before 90 days of curing, sample 16FC25 also achievedthe same strength as the control. The strength of concrete using thecoarsest particle, 18CC25, was only 84.4% of the control concrete at 180days. These results which are summarized in Table 9, again show that thestrength of fractionated fly ash concrete depends on the particle sizeand their distribution within the fractionated fly ash. Fly ashfractions containing smaller size particles demonstrated higher rates ofcompressive strength gain.

                                      TABLE 9                                     __________________________________________________________________________    Percentage Compressive Strength of the                                        Fractionated Dry Bottom boiler Fly Ash Concrete Over Time                     (25% Replacement)                                                             Sample                                                                             Percentage Compressive Strength (%)                                      No.  1-d 7-d  14-d 28-d 56-d 90-d 180-d                                       __________________________________________________________________________    CCCC 2157                                                                              6237 7141 8157 8707 9195 10161                                       13FC25                                                                             74.2                                                                              88.0 96.6 101.3                                                                              104.8                                                                              107.2                                                                              109.9                                       15FC25                                                                             71.8                                                                              86.1 93.4 96.3 100.9                                                                              104.9                                                                              106.2                                       16FC25                                                                             68.6                                                                              82.8 88.8 92.2 97.5 102.6                                                                              103.6                                       18FC25                                                                             64.4                                                                              78.2 84.4 87.5 90.8 93.8 96.2                                        18CC25                                                                             63.4                                                                              74.4 79.4 77.8 82.9 84.4 84.4                                        CWET25                                                                             65.1                                                                              78.9 84.3 88.4 93.2 93.5 93.2                                        __________________________________________________________________________

With 35% replacement of cement by fly ash in concrete, the compressivestrengths were lower than those for 15% and 25% replacement, especiallyat the early ages (FIG. 3C). The compressive strength of fractionatedfly ash concrete at 1 day varied from 851 psi to 1460 psi, moving fromcoarse to fine particle size ranges. Most strengths of fractionated flyash concrete were lower than the control concrete at all ages. Thenotable exception was the sample with the finest particle size range offly ash, 13FC35. The strength of sample 13FC35 varied from 1460 psi at 1day to 10788 psi at 180 days, or from 67.6% to 106.2% of the control.The strength of fly ash concrete with 35% replacement was as high as thecontrol strength by 90 days with the 13F fly ash fraction. With theoriginal feed of wet bottom boiler fly ash, CWET35, had a compressivestrength at 180 days about 90% of the control strength.

Replacement of 50% of cement with wet bottom boiler fly ash yieldedconcrete of much lower compressive strength (FIG. 3D). The replacementof cement with fly ash to 50% by weight of cementitious materials gavevery low strength at 1 day. The compressive strength at 1 day variedfrom 484 psi to 733 psi, or from 22.4% to 34.0% of the control strength.After 180 days of curing, all of the fractionated fly ash concretesamples were of lower compressive strength than the control.

Although the amount cement in each fly ash concrete was only half of thecontrol sample, some of fly ash concretes still gave a reasonablestrength result. Sample 13FC50 has compressive strength of 9672 psi, or95.2% of the control, at 180 days. The strengths of samples 15FC50 and16FC50 were 88.2% and 80.8% of the control concrete, respectively.

Example 3

Workability of Fractionated Fly Ash Concrete

Slump test results were obtained for fractionated fly ash concrete. Theslump was usually higher when fly ash was used, in agreement with Ukitaet al. (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans inConcrete, SP-114, American Concrete Institute, Detroit, pp. 219-240).Incorporation of fly ash in concrete often improves workability, whichin turn reduces the amount of water required compared to conventionalconcretes (Lane and Best, 1982, supra; ACI 226 1987, "Use of Fly Ash inConcrete," ACI 226-3R-87, ACI J. Proceedings 84:381-409; Yamato andSugita, 1983, Fly Ash, Silica Fume, Slag, and Other Mineral By-Productsin Concrete, SP-79, American Concrete Institute, Detroit, pp. 87-102).

The results of this experiment show that only the finest fly ash reducedthe workability of fresh concrete, especially when high quantities offly ash were used. The other sizes of fly ash increased slump. Theseobservations are explainable by the fact that, since the weight of flyash was kept constant, the finer particle fly ash with greater surfacearea required more water to maintain the same workability as coarsersizes of fly ash.

With 50% fly ash of the finest particle size in the cementitiousmaterials, the fly ash concrete samples of dry and wet bottom boiler flyashes, 3FC50 and 13FC50, were less workable than those of the controlconcrete, which had a slump of about 5 cm. The slump of fly ash concretefrom the original feed fly ashes was slightly higher than the control.For the original feed fly ashes, samples from the dry bottom boiler flyash, CDRY, were found to be more workable than those from the wet bottomboiler fly ash, CWET. This may be because the particle sizes of the drybottom boiler fly ash were larger than those of the wet bottom boilerfly ash. With the same amount of fly ash in the mix, the coarsestparticle sizes, 1CC and 18CC, had a little lower slump than the originalfeed fly ash concrete.

Example 4

Setting Time of Fractionated Fly Ash-Cement Paste

The setting times of fly ash-cement paste increased with the increasedthe amount of fly ash in the paste. The same results were also reportedby Ravina (1984, Concrete Int'l: Design and Construction 6:35-39),Meinlinger (1982, Concrete Int'l: Design and Construction 11:591-603),and Lane and Best (1982, supra). The initial and final setting timeswere slightly changed with the 15% replacement of the fractionated dryand wet bottom boiler fly ashes. The initial setting time offractionated fly ash-cement paste was about 2 h and 55 min, while thesetting time of the cement paste was 2 h 40 min. The final setting timesof fly ash-cement paste with 15% replacement (dry or wet bottom boilerfly ash) were about 25 minutes longer than the final setting time of thecement paste.

With 25% replacement, the initial setting times increased 20 to 35minutes from the initial setting time of cement paste, depending on theparticle size of fly ash. The fine particle size fly ash fraction cementpaste seemed to set taster than the paste using coarse fly ashfractions. For the dry bottom boiler fly ash, the initial and finalsetting times of sample 3F were 3 h and 6 h, respectively while theinitial and final sets of the sample 1C were 3 h, 10 min and 6 h, 10min, respectively. The setting time of the sample using the originalteed of wet bottom boiler fly ash was slightly shorter than that of thedry one.

When the replacement of fly ash increased to 35% by weight ofcementitious materials, the setting times increased compared to sampleswith 15% and 25% replacement. The initial setting times of fractionatedfly ash-cement paste were usually about 3 h, 20 min. The final settingtimes of samples from the fractionated dry bottom boiler fly ashes werelonger than those for the fractionated wet bottom boiler fly ashes byabout 20 to 30 minutes. With 35% replacement with the fractionated drybottom boiler fly ashes, the final setting times were about 1 hourlonger than the final setting time of the control cement paste. With thesame replacement of the fractionated wet bottom boiler fly ashes, thefinal setting times were about 40 minutes longer than for the cementpaste.

With 50% of fly ash in the fly ash-cement paste, the initial settingtimes of the fractionated dry bottom boiler fly ashes were about 1 hourlonger than the setting time of the cement paste control. In general,the initial and final setting times of the fractionated wet bottomboiler fly ash samples were shorter than those of the dry bottom boilerfly ash samples. This may have been due to the fact that thefractionated wet bottom boiler fly ashes have higher CaO content thanthe dry bottom boiler fly ashes. The CaO content of the originated ofwet and dry bottom boiler fly ashes was 6.89% and 2.41%, respectively.Since CaO can react with water and set like cement, extra CaO may haveresulted in setting of the fractionated wet bottom boiler fly ash pastesset faster than the dry bottom boiler fly ash paste.

Example 5

Compressive Strength of Concrete Containing Fractionated Fly Ash andSilica Fume

Concrete mixtures containing the finest fraction of both dry bottomboiler and wet bottom boiler fly ash (fractions 3F and 13F,respectively) or silica fume were prepared to compare the independenteffects of each of these components on the rate of compressive strengthgain for concrete. These agents were used as a 15% or a 25% replacementfor cement in the cementitious materials of the concrete.

Superplasticizer (Daracem-100) was added to reduce the amount of waterrequired for the mixture. In the mixture containing 25% fly ash orsilica fume as a replacement for cement, 10 ml per pound of cementitiousmaterials (cement and silica fume or fly ash) of superplasticizer wasused.

A control sample (labeled CSF) was prepared, which contained neither flyash nor silica fume. The labels CSF15 and CSF25 refer to concretecontaining 15% and 25%, respectively, silica fume for cement. The labelsC3F15, C3F25, C13F15, and C13F25 refer to samples 3F and 13F,respectively, containing 15% or 25% fly ash for cement, respectively.

Table 10 and FIG. 4A shows the compressive strength over time ofcompositions in which fly ash or silica fume replace 15% or 25% of thecement.

                                      TABLE 10                                    __________________________________________________________________________    Compressive Strength of the Fractionated Fly Ash                              and Silica Fume Concrete                                                      (15% and 25% Replacement)                                                     Percentage Compressive Strength (%)                                           Sample                            Slump                                       No.  1-d 7-d  14-d 28-d 56-d 90-d (cm)                                        __________________________________________________________________________    CSF* 1912                                                                              6352 7346 7881 8645 9322 23                                          CSF15                                                                              122.1                                                                             113.0                                                                              105.7                                                                              101.6                                                                              109.9                                                                              99.6 1                                           CSF25                                                                              139.9                                                                             104.9                                                                              101.8                                                                              101.9                                                                              98.4 97.9 0                                           C3F15                                                                              63.6                                                                              92.2 96.1 99.2 104.5                                                                              107.5                                                                              21                                          C3F25                                                                              63.4                                                                              94.0 98.7 109.7                                                                              113.1                                                                              112.9                                                                              20                                          C13F15                                                                             101.7                                                                             106.8                                                                              106.2                                                                              110.9                                                                              114.0                                                                              112.5                                                                              16                                          C13F25                                                                             93.2                                                                              95.9 98.6 108.6                                                                              113.6                                                                              115.3                                                                              12                                          __________________________________________________________________________     *The values for the control are the compressive strength in psi; all othe     values are the percentage of control compressive strength at that time.  

In these mixtures, cementitious materials, sand, course aggregate,water, and superplasticizer are held constant. Therefore, theconsistency and compressive strength of the concrete depends on thecomponents of the cementitious materials, i.e., the fly ash or silicafume and cement.

The data show that concrete containing 25% silica fume had no slump,while control concrete had a 23 cm slump. Since silica fume is very fineparticle material, it has more surface area than cement on a per weightbasis. In general, when the mix proportion of concrete is maintainedconstant, the mix with silica fume (powder) requires additional water toachieve a satisfactory slump.

Concrete containing fly ash also demonstrates a lower slump thancontrol. Although fly ash concrete usually has more slump than control,the finest fractions of fly ash demonstrate the opposite behavior. Thus,the presence of these fractions in the concrete reduces the workabilityof the material. However, it is clear that fly ash does not serve toeliminate slum at the desired content in concrete, in contrast to silicafume.

The compressive strength of the control concrete varied from 1912 psi atday 1 to 9322 psi at day 180. Within 7 days, the compressive strength ofCSF was 6352, which is considered a high strength concrete (ACICommittee 363, 1990, ACI Manual of Concrete Practice Part I, AmericanConcrete Institute, Detroit).

The compressive strength of CSF15 and CSF25 (which contain silica fume)after 1 day were 2335 and 2675 psi, respectively, or 22.1% and 39.9%stronger than control. Concrete containing silica fume achieved earlycompressive strength gains. This behavior can be attributed to bothpacking and pozzolanic effects. Because the particle sizes of silicafume are very small, they fill the voids of the concrete matrix and makeconcrete denser and more compact after casting. During the curingperiod, the pozzolanic reaction by silica fume takes place at a fasterrate than observed for fly ash probably because it is much finer. After28 days, however, the rate of strength gain of silica fume concreteslows, and its absolute strength falls below that of the control. Thepercentage of control strength of high strength silica fume concretewith 25.5% replacement goes from 139.9% on day 1 to 97.9% on day 90.

High strength concrete made from the finest fractions of fly ash behavesdifferently. Early strength gains by fly ash concrete occur much moreslowly than with control. With 15% replacement of 3F fly ash, thecompressive strength of fly ash concrete varies from 1216 psi at day 1to 10023 psi at day 90, a shift form 63.4% of control strength to 107.5%of control strength. This trend was observed with other fly ashconcretes tested as well. The strength variation of high strength flyash concrete with 25% replacement of 13F fly ash varies from 1782 psi atday 1 to 10748 psi at day 90. The values for the 25% fly ash concretewere lower during the first 14 days than the corresponding compressivestrength values of the 15% replacement concrete. However, after 90 days,the concrete with a greater percentage of fly ash is stronger.

The expected compressive strength of the C3F15 sample at day 1 is 80% ofcontrol. The lower value actually observed (63.4%) may be due to thehigh dose of superplasticizer used, which was about three times higherthan the manufacturer's recommendation. A high dose of superplasticizertends to retard the setting of cement, resulting in lower compressivestrength early on. This effect was not pronounced with the fraction ofwet bottom boiler fly ash, 13F.

After 7 days of curing, the rate of compressive strength gain of the flyash concrete samples returned to the expected level. The fly ashconcrete was considered to be high strength after seven days of curing,since at this time the compressive strength of the samples is over 6000psi.

Before day 7, the highest strength is found in the samples containingsilica fume. After 14 days, samples CSF15 and C13F15 have comparablestrengths, at about 7800 psi. At day 28 of curing, high strengthconcretes using fly ash as a replacement produced stronger concrete thanfor either the control or the silica fume concrete. The strength ofsamples C13F15, C13F25, and C3F25 were 8740 psi, 8561 psi, and 8648 psi,respectively. As the concrete ages, the fly ash continues to contributeto values for compressive strength that are higher than control values.At about 90 days, the compressive strengths of fly ash concrete are muchhigher than control.

It is interesting to note that the compressive strength of concrete madewith silica fume (containing either 15% or 25%) have almost the samestrength as the control strength at 90 days. These values range from9100 psi to 9300 psi. The results of this experiment clearly show thatwhile the silica fume contributes to a more rapid initial gain incompressive strength, after about a week the rate becomes much slower.The data suggest that a mixture containing silica fume, for rapidstrength gain, and fly ash, for long term compressive strength gain, maybe particularly advantageous.

Example 6

Effect of Fractionated Fly Ashes on the Strength of Mortar

In addition to the study of fractionated fly ash concrete, mortar wasalso tested. The fractionated fly ashes from the dry and wet bottomboilers were used as a replacement for cement in the mortar at 15%, 25%,and 50% by weight of cementitious (cement+fly ash) materials. The waterto cementitious materials ratio was kept constant at 0.5. Control mortarwithout any fly ash replacement, using the same mix proportion and thesame water to cementitious materials ratio, was also mixed and cast. Themix proportion is shown in Table 1. After casting 24 hours, the 2"×2"×2"cube samples were removed from the mold and cured in saturated limewater prior to testing. The compressive strength of samples were testedafter 1, 3, 7, 14, 28, 56, 90, and 180 days of aging.

CF is the control sample. Samples "DRY" and "WET" were the mortars withthe original feed of dry and wet bottom boiler fly ashes, respectively.The numbers "15", "25", and "50" stand for the percentage of cementreplaced by fly ash in the mortar. Fractionated fly ash samples aredescribed above (see Table 3). For example, the 3F15 sample is fly ashmortar using 3F fly ash as a substitute for cement to 15 percent byweight of cementitious materials. Likewise, 6F15 is the fly ash mortarusing 6F fly ash as a replacement for cement 15 percent by weight ofcementitious materials.

The relationship between the compressive strength of fractionated flyash mortar and age is shown in FIG. 5A (dry bottom boiler fly ash) andFIG. 5B (wet bottom boiler fly ash).

As expected, and as observed in the tests with concrete mixtures, theearly age strengths of fly ash mortar were lower than the controlmortar. With 15% replacement with fractionated fly ash, the compressivestrength was no more than 80% of the control mortar strength at 1 day.The compressive strengths of fractionated fly ash mortars graduallyincreased with age, depending on the average and range of volumes of flyash particles. As observed with concrete, the strength of fly ash mortarincreased with the decrease in the particle size range of fractionatedfly ash. Compressive strength varied from 2290 psi for coarse particlesto 2666 psi for fine particles. At all curing ages, the lowestcompressive strength was found in samples containing coarse particles offly ash (1C15 and 18C15).

Up to 14 days, the compressive strengths of all fly ash mortars werelower than the control, except for the samples containing the finest flyash fractions (3F15 and 13F15). The compressive strengths of samples3F15 and 13F15 at 14 days were 7968 psi and 7925 psi, respectively.These strengths represent 101.1% and 100.5% of the control strength.After 180 days of curing, all samples of fractionated fly ash mortardemonstrated greater strength than the control sample, except samples1C15 and 18C15, which were made up of the coarsest particles of eachtype of fly ash. The compressive strengths of 1C15 and 18C15 were 93.6%and 92.7%. respectively, of the control at 180 days.

In summary, these results show that at the same age and for the sametype of fly ash, the finer the particle size of fly ash in the mortar,the higher the compressive strength of the mortar will be.

It was noted that the strength of the mortar made from thenon-fractionated wet bottom boiler fly ash (WET15) was slightly higherthan that from the non-fractionated dry bottom boiler fly ash. Althoughthe non-fractionated wet bottom boiler fly ash could be expected to havea greater glassy phase than non-fractionated dry bottom boiler fly ash,preliminary state-of-the-art X-ray diffraction results could notdistinguish a significant difference in the glassy phase. Thisobservation suggests that both fly ashes have about the same degree ofglassy phase. As will be show below the difference in compressivestrength between these non-fractionated samples correlates with thefineness modulus of the samples.

The compressive strength of fractionated fly ash mortar with 25%replacement was somewhat lower than that with 15% replacement (FIGS. 6Aand B), although the trend was the same. All of the early strengths ofthe fractionated of dry bottom boiler fly ash mortars were lower thanthe control mortar for up to 28 days. With 25% replacement, the strengthof mortar from the original feed (dry or wet bottom boiler fly ash) wasonly about 30% of the control strength at 1 day. For replacement withthe fractionated wet bottom boiler fly ashes, most of fly ash mortarsamples demonstrated lower compressive strength than the controlstrength at the age of 28 days, except for sample 13F25. The compressivestrength of 13F25 was 9112 psi, or 100.2% of the control, at 28 days.Replacement with the original feed of dry and wet bottom boiler flyashes yield compressive strengths of 7821 psi and 8031 psi,respectively, or 86% and 88.3%, respectively, of the control mortar at28 days. As noted above, the compressive strength of mortar from theoriginal feed of wet bottom boiler fly ash was slightly higher than themortar made from the original feed of dry bottom boiler fly ash. Thecompressive strength of fly ash mortar containing 25% coarse fly ashes,i.e, 1C25and 18C25 were 83.4% and 91.1% of control, respectively at theage of 180 days.

Thus, for both types of fly ash, the compressive strength offractionated fly ash mortar increased with the decrease of fly ashparticle size. After the age of 180 days, most of fly ash mortarsachieved the same or higher compressive strength than the control,except for mortar made with the coarsest particle size distributions(11F, 1C, and 18C) of fly ash. The original feed of fly ash required 180days of curing to gain the same compressive strength as the control. Theresults further demonstrate that the use of fine fractionated fly ashincreases the rate of pozzolanic activity. The finer the particle sizesin the fly ash fraction, the greater the rate of the strengthdevelopment.

With 50% replacement of fly ash in the mix, the early strengths of flyash mortar were very low (FIGS. 7A and 7B). All strengths offractionated fly ash mortars at 1 day were less than 50% of the control.The compressive strengths of fractionated fly ash mortars at 1 dayvaried from 711 psi to 1322 psi, depending on the particle size range ofthe fly ash fraction. The percentage compressive strength of sample 3F50varied from 46.4% at 1 day to 81.6% at 180 days. The respectivecompressive strengths of the original feed dry and wet bottom boiler flyash mortar samples were 26.2% and 30.2% of the control mortar. For theoriginal feed of dry bottom boiler fly ash sample, DRY50, thecompressive strength was 747 psi at 1 day, and increased to 7642 psi at180 days. In general, as was noted above, the compressive strength ofthe original feed of wet bottom boiler fly ash was higher than that ofdry bottom boiler fly ash. After 180 days, all of the fractionated flyash mortar samples demonstrated lower compressive strength than thecontrol. The graphs of FIGS. 10A and 10B suggest that the mortar samplesmade with fine fly ashes, i.e., 3F50, 6F50, 14F50, and 15F50, continuedto gain strength after 180 days. According to Hensen (1990, Cement andConcrete Research 19: 194-202), the pozzolanic activity of fly ashcontinues for up to 3 years after casting concrete or mortar.

Example 7

Compressive Strength Is lndependent of Median Particle Diameter andTotal Surface Area

The relationship between compressive strength and median diameter of thefractionated dry bottom boiler fly ash is shown in FIG. 8A-8D. It may beobserved that there is little difference in compressive strengthexhibited by the fractionated fly ash samples 11F (15.69 microns) and 1C(39.45 microns) when the median diameter is above 15 microns, withcuring times up to 56 days. However, as the median diameter of thefractionated fly ash becomes less than 15 microns, differences areobserved in the compressive strengths exhibited by the concrete samplesprepared with these different fractionated fly ashes. These are observedalter seven days of curing. At one day of curing, the compressivestrength exhibited by the concrete appears to be independent of themedian particle diameter.

Similar results are observed with the concrete samples prepared with thefractionated and non-fractionated wet bottom boiler fly ash, as shown inFIG. 9A-9D. The compressive strength of the fractionated fly ashconcrete 18F and 18C, with the largest particle sizes, containingdifferent amounts of fly ash replacement of cement with fly ash remainessentially constant with curing time up to 56 days. Below the 10 micronlimit, the compressive strength again increases with a decrease in themedian particle diameter after seven days of curing.

However, when one examines the relationship between compressive strengthexhibited by the non-fractionated dry and wet bottom boiler fly ashesand their median particle diameter, there is deviation of thecompressive strength point for the non-fractionated fly ash sample fromthe points obtained with the fractionated fly ash samples. Thisdeviation is not as significant for the non-fractionated dry bottomboiler fly ash (median diameter 13.73 microns) as it is for thenon-fractionated wet bottom boiler fly ash (median diameter 6.41microns). When a broad distribution of sizes of non-fractionated fly ashis used in the concrete, the relationship between the compressivestrength and the median particle diameter is different than thatobtained when fractionated fly ash, with a narrow particle sizedistribution, is used.

The relationship between the compressive strength of thenon-fractionated dry bottom boiler and wet bottom boiler fly ash andtheir total surface areas as measured by Blaine fineness also deviatesfrom that obtained with the fractionated fly ashes. In a comparison ofthe compressive strengths exhibited by the non-fractionated dry bottomboiler fly ash (Blaine 3235 cm² /g, median diameter 13.73 microns) andthe 10F fraction (Blaine 2028 cm² /g, median diameter 12.12 microns)(see FIG. 8A-8D), the 10F concrete sample in general exhibits a greatercompressive strength than the non-fractionated dry bottom boilerconcrete sample, even though its total surface area is significantlyless than the total surface area of the non-fractionated fly ashconcrete as measured by Blaine fineness.

The difference in compressive strength is more significant when thecompressive strength of the non-fractionated wet bottom boiler fly ashconcrete (Blaine 5017 cm² /g, median diameter 6.41 microns) is comparedwith the 16F fraction concrete sample (Blaine 5171 cm² /g, mediandiameter 5.5 microns). Here the Blaine fineness is comparable and mediandiameter is not significantly different (see FIG. 9A-9D) to thenon-fractionated wet bottom boiler fly ash sample.

These results indicate that both the size of the fly ash particles andthe distribution of the particle sizes must be considered in definingcompressive strength, especially when the predominant particle sizes inthe fly ash is below the 10 micron to 15 micron range.

The compressive strength development resulting from the reaction betweenthe cement and fly ash appears to be primarily particle volume dependentand not surface area dependent. After seven days of curing the concrete,the smaller particles in the fractionated fly ashes with the smallestparticle sizes have probably reacted completely. Thus, the compressivestrengths of these fractionated fly ash-concrete samples are measured tobe greater than those containing the larger particle sizes.

If the compressive strength development was primarily surface aredependent, compressive strength differences would be observed even afterone day of curing. An examination of Table 5 shows the large variationin surface area, as represented by the Blaine fineness, exhibited by thefractionated fly ashes. Yet, the concrete samples containing thesefractionated fly ashes show virtually no differences in the compressivestrength after one day. In fact, the dry bottom boiler and wet bottomboiler fractionated fly ash concrete 11F and 1c, and 18F and 18C,respectively, containing the largest particle sizes, show no compressivestrength difference even after 56 days of curing. In contrast, theinfluence of particle sizes on compressive strength is readily observedfor the fractionated fly ashes with smaller particle sizes.

Example 8

Fly Ash Concrete Strength Model

Based on the observations disclosed above, a fly ash concrete strengthmodel is proposed. This model considers the contribution to compressivestrength of cement and fly ash in concrete or mortar at any given timepoint. Although the specific model and equation are derived for concretecompositions, the important variables and relationships of the variabledescribed in the equation broadly apply to any hardenable mixture,whether concrete or mortar, that contains fly ash.

Two factors determine strength at time O: the amount of cement that ispresent, and any packing effect mediated by fly ash. With time,pozzolanic activity of fly ash with CaO created by the cement leads to agreater increase in compressive strength. Thus, the model includes thecontribution of fly ash pozzolanic activity over time as well as packingeffects mediated by the fly ash.

To simplify the calculation, the present example maintained thecementitious materials (cement and fly ash) in a constant ratio towater, sand (fire aggregate), coarse aggregate, etc. Compressivestrength of fly ash concrete was thus predicted as a percentage ofcontrol strength.

The compressive strength of control concrete was obtained empiricallyfrom the concrete that has the same mix proportions, and settingconditions such as the fly ash concrete, i.e., the water to cementitiousmaterials ratio, curing condition, type of aggregates, and othervariable were kept constant. The difference between the control and flyash concrete mix was that all of the cementitious materials in thecontrol concrete were cement.

Most importantly, the critical measure of the contribution of fly ash tostrength gain is a parameter termed the fineness modulus of fly ash.

The variables of the equation to predict compressive strength of fly ashconcrete are thus fineness modulus of the fly ash, age of concrete, theratio of cement to fly ash, and the strength of the control concrete.

Fineness Modulus of Fly Ash (FM)

Fineness modulus of fly ash (FM) in this Example is defined as thesummation of the percentage of fly ash that retained on the followingsieve sizes: 0, 1, 1.5, 2, 3, 5 10, 20, 45, 75, 150, and 300 microns. Ingeneral, very little fly ash retained on the sieve with the opening sizelarger than 600 microns. The fineness modulus of fly ash was usedwithout units. The value of fineness modulus is a measure of how thedistribution of particle sizes s of fly ash from one sample compare toother fly ash samples.

The fineness modulus of fractionated dry bottom boiler and wet bottomboiler fly ashes are presented in Tables 10 and 11, respectively. Thefineness modulus of fractionated fly ashes was between 300 to 900. Flyash 13F has the lowest fineness modulus (the finest fly ash), and 1C hasthe highest fineness modulus (the coarsest fly ash).

                  TABLE 11                                                        ______________________________________                                        Fineness Modulus of the Fractionated Dry                                      Bottom boiler Fly Ashes                                                       Sieve                                                                         Opening  Percent Retained (%)                                                 (Micron) 3F     6F       10F  11F    1C   DRY                                 ______________________________________                                        300      0      0        0    0      1    0                                   150      0      0        0    0      4    1                                   75       0      0        0    0      17   8                                   45       0      0        0    1      44   20                                  20       0      0        5    20     80   40                                  10       0      5        60   82     99   55                                  5        10     53       94   96     100  70                                  3        35     78       96   97     100  80                                  2        55     83       97   98     100  87                                  1.5      75     90       97   100    100  92                                  1        93     94       98   100    100  95                                  0        100    100      100  100    100  100                                 FM       368    503      647  694    845  648                                 ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                        Fineness Modulus of the Fractionated Wet                                      Bottom boiler Fly Ashes                                                       Opening  Percent Retained (%)                                                 (Micron) 13F    15F      16F  18F    18C  WET                                 ______________________________________                                        300      0      0        0    0      0      0                                 150      0      0        0    0      3      2                                 75       0      0        0    0      10     5                                 45       0      0        0    0      30     10                                20       0      0        0    6      70     20                                10       0      3        10   39     96     35                                5        6      30       49   80     100    55                                3        35     56       73   86     100    70                                2        49     69       82   89     100    80                                1.5      68     82       88   93     100    88                                1        82     90       92   94     100    97                                0        100    100      100  100    100    100                               FM       340    430      494  587    809    562                               ______________________________________                                    

The results disclosed above demonstrate that when fly ash is used toreplace an equal amount by weight of cement in concrete or mortar,compressive strength of the concrete or mortar is inversely proportionalto the fineness modulus of fly ash.

Fineness modulus of fly ash provides a more predictive measure of thecompressive strength of concrete than other measures of fineness, suchas the Blaine fineness and the residue on sieve No. 325. Concrete madeup with 10F fly ash (Blaine 2028 cm² /g) gave higher strength thanconcrete containing an equal amount of the original teed of dry bottomboiler fly ash (Blaine 3235 cm² /g), which is the opposite of theexpected result based solely on the observations described above (thatfiner fly ash particles give greater compressive strength results) andthe values of Blaine fineness (greater surface area per gram of materialis indicative of greater fineness).

Similarly, the 45 micron sieve test lacks predictive value. Fly ashes3F, 6F, and 10F, which have zero value retained on sieve on sieve No.325, yield remarkable differences in compressive strength when used inconcrete or mortar.

Thus, neither method is suitable to provide an indication of the effecton compressive strength of fractionated fly ash. In contrast, using thefineness modulus of fly ash provides reliable information concerning thecompressive strength of fly ash concrete and mortar.

The irrelevance of Blaine fineness, as well as the correlation to mediandiameter, to compressive strength of concrete is evident in theexperimental data. For example, when fractionated wet bottom boiler flyash replaced 35% of cement in cementitious materials in concrete, therewas a clear relationship between median diameter and compressivestrength at all time points, whereas compressive strength wasindependent of Blaine fineness at a value greater than about 4000-5000cm² /g. Similar data were observed for concrete in which dry bottomboiler fly ash was used for 35% of cementitious materials. In the lattercase, compressive strength became independent of Blaine fineness aboveabout 2000 cm² /g.

Fly Ash Concrete Strength Predictive Formula

A specific formula for predicting fly ash concrete strength is in theform of:

    σ(%)=σc+σFA                              (1)

in which σ(%) is the percentage compressive strength of fly ash concretecompared to control concrete;

σ_(c) is the percentage compressive strength of concrete contributed bycement in the concrete mix, which is equal to:

    σ.sub.c =0.010C.sup.2                                (2)

where C is the percentage of cement in the cementitious materials;

σ_(FA) is the contribution to strength by the pozzolanic reactionbetween fly ash and cement at any age, and can be given as:

    σ.sub.FA =A+(B/FM)ln(T)                              (3)

where A is a constant for the packing effect contribution of fineness offly ash to the strength of concrete. For dry and wet bottom boiler flyashes, this constant can be expressed as:

    A=6.74-0.00528FM                                           (4)

in which FM is the fineness modulus of fly ash.

B in formula (3) above is the value for the pozzolanic activity betweenfly ash and cement for any mix proportion or ratio. B depends on the flyash content in the mix. With higher fly ash content, this constant ishigher; it decreases as the percentage of fly ash in the mixturedecreases. For fly ash content between 10% to 50% by weight ofcementitious materials, the constant B can be expressed by the formula:

    B=[1685+126C-1.324C.sup.2 ]                                (5)

The value of T in equation (3) is the age of concrete in days. FIG. 10graphically depicts equation B for 10% to 50% replacement of cement withfly ash.

Thus, the final form of the formula for predicting fly ash concretestrength is:

    σ(%)=0.010C.sup.2 +[6.74-0.00528FM]+{B/FM[ln(T)]}    (6)

When the fly ash content in a concrete mix is between 10% to 50%,equation (6) can also be expressed as:

    σ(%)=0.010C.sup.2 +[6.74-0.00528FM]+{(1685+126C-1.324C.sup.2)/(FM)[ln(T)]}  (7)

After the compressive strength of fly ash concrete is determined as apercentage of compressive strength of control concrete without fly ash,the actual compressive strength of fly ash concrete can be determined bymultiplying the strength at same control age with the percentcompressive strength of fly ash concrete. The age of the concrete, T, isvaried from 1 day to 1000 days. After 1100 days (3 years), the strengthof fly ash concrete does not increase significantly (Hensen, 1990,supra).

Example 9

Prediction of Compressive Strength of Fractionated Fly Ash Concrete

Regardless of the type of boiler use for burning coal to produce flyash, equation (7) gives a very close prediction of compressive strengthof feed and fractionated dry bottom boiler and wet bottom boiler fly ashconcrete. FIGS. 11 A-D show the correlation between experimentalobservation (data points) and prediction with the model (line) forcompressive strength over time of concrete containing 15%, 25%, 35% and50% 6F dry bottom boiler fly ash. The predictions of compressivestrengths of the fractionated 16F fly ash concrete using the equationare shown in FIGS. 12A-D. Equation (7) accurately predicted compressivestrength for a given amount of fly ash for all of the fractionsdescribed above.

The present invention is not to be limited in scope by the specificembodiments describe herein. Indeed, various modifications of theinvention in addition to those described herein will become apparent tothose skilled in the art from the foregoing description and theaccompanying figures. Such modifications are intended to fall within thescope of the appended claims.

Various publications are cited herein, the disclosures of which areincorporated by reference in their entireties.

What is claimed is:
 1. A high early strength concrete consistingessentially of about 1 part by weight cementitious materials, about 1 toabout 3 parts by weight fine aggregate, about 1 to about 5 parts byweight coarse aggregate, and about 0.35 to about 0.6 parts by weightwater, wherein the cementitious materials comprise from about 10% toabout 35% by weight fly ash and about 65% to about 90% by weight cement,wherein the fly ash comprises at least 99% of particles having aparticle size of less than 20 microns and having a fineness modulus ofless than about 503, wherein the fineness modulus is calculated as a sumof the percent of fly ash particles having a size greater than 0, 1,1.5, 2, 3, 5, 10, 20, 45, 75, 150, and 300 microns.
 2. The concrete ofclaim 1 wherein the fly ash has a fineness modulus of less than about350.
 3. The concrete of claim 1 wherein the fine aggregate comprisessand.
 4. The concrete of claim 1 wherein the fine aggregate comprisessand and fly ash, wherein a ratio by weight of sand to fly ash is fromabout 4:1 to about 1:1, and the fly ash has a fineness modulus of lessthan about 600, wherein the fineness modulus is calculated as the sum ofthe percent of fly ash retained on sieves of 0, 1, 1.5, 2, 3, 5, 10, 20,45, 75, 150, and 300 microns.
 5. The concrete of claim 1 furthercomprising silica fume.
 6. The concrete of claim 1 further comprisingglass fibers.
 7. A high early strength mortar consisting essentially ofabout 1 part by weight cementitious materials, about 1 to about 3 partsby weight fine aggregate, and about 0.35 to about 0.6 parts by weightwater, wherein the cementitious materials comprise from about 10% toabout 35% by weight fly ash and about 65% to about 90% by weight cement,and wherein the fly ash comprises at least 99% of particles having aparticle size of less than 20 microns and having a fineness modulus ofless than about 503, wherein the fineness modulus is calculated as thesum of the percent of fly ash particles having a size greater than 0, 1,1.5, 2, 3, 5, 10, 20, 45, 75, 150, and 300 microns.
 8. The mortar ofclaim 7 wherein the fly ash is wet bottom boiler fly ash having afineness modulus of less than about
 350. 9. The mortar of claim 7wherein the fine aggregate comprises sand.
 10. The mortar of claim 7wherein the fine aggregate comprises sand and fly ash, wherein ratio byweight of sand to fly ash is from about 4:1 to about 1:1, and the flyash has a fineness modulus of less than about 600, wherein the finenessmodulus is calculated as the sum of the percent of fly ash retained onsieves of 0, 1, 1.5, 2, 3, 5, 10, 20, 45, 75, 150, and 300 microns. 11.The mortar of claim 7 further comprising silica fume.
 12. The mortar ofclaim 7 further comprising glass fibers.